BEIRUT (AP) -- Syria said Monday it is prepared to hold talks with armed rebels bent on overthrowing President Bashar Assad, the clearest signal yet that the regime is growing increasingly nervous about its long-term prospects to hold onto power as opposition fighters make slow but persistent headway in the civil war.

Meanwhile, the umbrella group for Syrian opposition parties said it had reversed a decision to boycott a conference in Rome being held to help drum up financial and political support for the opposition. Walid al-Bunni, a spokesman for the Syrian National Coalition, said the move came after a phone call between the group's leader, Mouaz al-Khatib, and U.S. Secretary of State John Kerry.

Al-Bunni told pan-Arab broadcaster Al-Arabiya the decision was made based on guarantees al-Khatib heard from western diplomats that the conference would be different this time. He did not elaborate. The boycott had put the group at odds with its Western backers.

The Syrian talks offer, made by Foreign Minister Walid al-Moallem during a visit to Moscow, came hours before residents of Damascus and state-run TV reported a huge explosion and a series of smaller blasts in the capital, followed by heavy gunfire.

State-run news agency SANA said there were multiple casualties from the explosion, which it said was a suicide car bombing. Britain-based activist group the Syrian Observatory for Human Rights said the explosions targeted a checkpoint, adding there were initial reports of at least five regime forces killed and several wounded.

The talks proposal marked the first time that a high-ranking regime official has stated publicly that Damascus would be willing to meet with the armed opposition. But al-Moallem did not spell out whether rebels would first have to lay down their weapons before negotiations could begin -- a crucial sticking point in the past.

The regime's proposal is unlikely to lead to talks. The rebels battling the Syrian military have vowed to stop at nothing less than Assad's downfall and are unlikely to agree to sit down with a leader they accuse of mass atrocities.

But the timing of the proposal suggests the regime is warming to the idea of a settlement as it struggles to hold territory and claw back ground it has lost to the rebels in the nearly 2-year-old conflict.

Opposition fighters have scored several tactical victories in recent weeks, capturing the nation's largest hydroelectric dam and overtaking air bases in the northeast. In Damascus, they have advanced from their strongholds in the suburbs into neighborhoods in the northeast and southern rim of the capital, while peppering the center of the city with mortar rounds for days.

Monday night's explosion struck about 800 yards (meters) from Abbasid Square, a landmark plaza in central Damascus. It was followed by several other smaller blasts thought to be mortar shells landing in various districts of the capital. The blasts and subsequent gunfire caused panic among residents who hid in their apartments.

On Thursday, a car bomb near the ruling Baath Party headquarters in Damascus killed at least 53 people, according to state media.

While the momentum appears to be shifting in the rebels' direction, the regime's grip on Damascus remains firm, and Assad's fall is far from imminent.

Still, Monday's offer to negotiate with the armed opposition -- those whom Assad referred to only in January as "murderous criminals" and refused to talk with -- reflects the regime's realization that in the long run, its chances of keeping its grip on power are slim.

Asked about al-Moallem's remarks, U.S. State Department spokesman Patrick Ventrell said the offer of talks was a positive step "in the context of them raining Scuds down on their own civilians." But he expressed caution about the seriousness of the offer.

"I don't know their motivations, other than to say they continue to rain down horrific attacks on their own people," Ventrell told reporters in Washington. "So that speaks pretty loudly and clearly."

If the Assad regime is serious, he said, it should inform the U.N. peace envoy, Lakhdar Brahimi of its readiness for talks. Ventrell said the regime hasn't done that yet.

Andrew Tabler, a fellow at the Washington Institute, called the offer "a sign of weakness."

"I think everybody knows, including Bashar Assad, that they (the regime) can't hang onto the whole country," Tabler said.

Mustafa Alani, an analyst with the Gulf Research Center in Geneva, said the regime has "reached the conclusion that they are heading toward a major defeat eventually, and this is the right time to negotiate."

"They are not losing miles every day, but they are losing substantial ground every day. So the regime is not genuine (in its offer) because it has changed, it's genuine because it is responding to a major shift in the balance of power on the ground," he added.

Alani cautioned, however, that the regime is also eager to keep the idea of talks alive in order to forestall any Western decision on arming the rebels. As long as the possibility of negotiations is still on the table, the U.S. and the European Union -- which have so far provided only non-lethal aid -- will be reluctant to open the flood gates on weapons for the opposition, he said.

"The whole regime tactic is to delay supplying arms, to buy time," Alani said. "The regime can show good will. Whether they're a viable partner or not is a different story."

It's also unclear who exactly the regime would sit across from at the negotiating table.

The dozens of armed groups across Syria fall under no unified command and do not answer to the Syrian National Council, which the West recognizes as the legitimate representative of the Syrian people.

At least one group offered a lukewarm response Monday to al-Moallem's proposal. Free Syrian Army chief Gen. Salim Idriss, said he is "ready to take part in dialogue within specific frameworks," but then rattled off conditions that the regime has rejected in the past.

"There needs to be a clear decision on the resignation of the head of the criminal gang, Bashar Assad, and for those who participated in the killing of the Syrian people to be put on trial," Idriss told Al-Arabiya TV.

He said the government must agree to stop all kinds of violence and to hand over power, stating that "as rebels, this is our bottom line."

Syria's 23-month-old conflict, which has killed more than 70,000 people and destroyed many of the country's cities, has repeatedly confounded international efforts to bring the parties together to end the bloodshed. Russia, a close ally of Assad and his regime's chief international advocate, offered Feb. 20, in concert with the Arab League, to broker talks between the rebels and the government.

With the proposal, which the Kremlin would be unlikely to float publicly without first securing Damascus' word that it would indeed take part, Moscow ratcheted up the pressure on Syria to talk to the opposition.

Russia has shielded Assad's government from U.N. action and kept shipping weapons to the military, but it is growing increasingly difficult to protect the regime as the violence grinds on.

Russian Foreign Minister Sergey Lavrov repeated his call Monday for Syria to negotiate with the opposition, saying before meeting al-Moallem that "the situation in Syria is at a crossroads now." He also warned that further fighting could lead to "the breakup of the Syrian state."

Past government offers for talks with the opposition have included a host of conditions, such as demanding that the rebels first lay down their arms. Those proposals have been swiftly rejected by both activists outside Syria and rebels on the ground.

Both sides in the conflict in recent weeks have floated offers and counteroffers to hold talks on the crisis.

In a speech in January, Assad offered to lead a national dialogue to end the bloodshed, but said he would not talk with the armed opposition and vowed to keep fighting. The opposition rejected the proposal.

This month, the SNC's al-Khatib said he would be open to discussions with the regime that could pave the way for Assad's departure, but that the government must first release tens of thousands of detainees. The government refused, and even members within the coalition balked at the idea of talks.

Speaking to reporters Monday in Cairo, al-Khatib accused the regime of procrastinating and said it had derailed his dialogue offer by not responding to the coalition's conditions.

"We are always open to initiatives that stop the killing and destruction, but the regime rejected the simplest of humanitarian conditions. We have asked that the regime start by releasing women prisoners and there was no response," he said. "This regime must understand that the Syrian people do not want it anymore."

The U.S.'s Kerry on Monday urged rebel leaders not to skip the Rome meeting and insisted that more help is on the way.

Kerry made a public plea at a joint news conference with British Foreign Secretary William Hague and also called al-Khatib "to encourage him to come to Rome," a senior U.S. official said. The official spoke on condition of anonymity because he wasn't authorized to speak publicly on the matter.

Meanwhile, the fighting inside Syria rages on.

The Observatory reported heavy clashes Monday near a police academy in Khan al-Asal just outside Aleppo.

Rebels backed by captured tanks launched an offensive on the facility Sunday. Observatory director Rami Abdul-Rahman said at least 13 rebels and five regime troops were killed.

In another part of Aleppo, rebels downed a military helicopter near the Mennegh airport, where there have been fierce clashes for months.

A video posted online by activists showed a missile being fired, a trail of white smoke and the aircraft going up in flames. Voices in the background shouted, "God is great!" as a man raised both hands in celebration.

The video appeared to be authentic and corresponded to other AP reporting.

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Associated Press writers Nataliya Vasilyeva in Moscow, Albert Aji in Damascus, Zeina Karam in Beirut, Matthew Lee in London, Bradley Klapper in Washington and Geir Moulson in Berlin contributed to this report.