BEIRUT (AP) -- Opposition forces targeted Damascus with mortars, a roadside bomb and a suicide attack on Sunday as they pressed ahead in their quest for the seat of President Bashar Assad's power.

Outside the capital, government troops battled rebels for the fifth straight day for control of a key highway. Both sides consider the fight for Damascus the most likely endgame in a nearly two-year-old civil war that has already killed more than 60,000 people.

Sunday's fighting was the heaviest in Damascus since the first rebel push into the capital in July. The rebels then managed to capture several neighborhoods, but were soon bombed out during a punishing government counteroffensive.

Since then, the rebels have threatened the heavily fortified capital from opposition strongholds around the city. Damascus, however, has been spared the kind of violence and destruction that has been seen in other major urban centers during the conflict.

Checkpoints on the main artery into the capital have changed hands several times since Wednesday when the latest rebel campaign for Damascus started. The road is strategically important because it leads to northern Syria and the regime uses it to move troops and supplies. Rebels cut the road off from Damascus with burning tires on Friday after seizing checkpoints from regime troops in fighting that brought the civil war within a mile of the heart of the capital.

A rebel fighter told The Associated Press that opposition forces on Sunday overran another roadblock, al-Adnan checkpoint in Jobar, northeast of Damascus. He spoke on condition of anonymity for security reasons.

But the Britain-based Observatory for Human Rights, an anti-regime activist group, said that while the fight for the highway continues, government troops regained control of the area on Sunday after using fighter jets to bomb rebel positions the day before.

A mortar that hit a Damascus street near Shabandar Square killed four people and injured several others, a government official told the AP on condition of anonymity because he was not allowed to brief the media.

State-run SANA news agency said a roadside bomb detonated at Arnous street, in the heart of Damascus, injuring several people. In another part of the city, a suicide bomber blew himself up. He was the only one who died in the blast in Rouken al-Deen neighborhood, the official news service said.

The rebels' latest push for Damascus is similar to rebel offensives in other Syrian cities. The opposition controls large swathes of land outside urban centers -- like Homs in central Syria and Deir el-Zour in the east, and even whole neighborhoods like in the northern city of Aleppo -- but cannot oust all government troops because of the regime's superior fire power.

The fighting has settled into a bloody stalemate and shows no signs of stopping, despite several tentative proposals from both sides to find a peaceful resolution to the conflict.

Also on Sunday, rebels fought a fierce battle with troops for control of military airport and artillery base that houses the Syrian army's 113th Brigade just outside the city of Dei el-Zour, the Observatory said. The Observatory said the rebels were using tanks they previously captured from the military in their assault on the regime's outposts in the city, which has the same name as the oil-rich province along Syria's border with Iraq that has been the scene of some of the heaviest fighting in the civil war.

Syria's Information Minister Omran al-Zoubi reiterated the regime's mantra that the only solution to the conflict is national dialogue in Syria, among Syrians, without foreign interference and in line with Assad's peace proposal that would keep him in charge of a reconciliation process.

"There is absolutely no other alternative," al-Zoubi told reporters after attending the enthronement of a new patriarch in Damascus. Syrian Deputy Foreign Minister, Faisal Mekdad, who was also at the ceremony, warned the West to stop interfering in Syrian affairs and called on opposition leaders to start talks with the regime without preconditions.

The opposition in January rejected Assad's initiative because it would keep him in power. The opposition and its Western backers insist Assad step down before any talks can begin. Late last month, the head of the Syrian National Coalition, Mouaz al-Khatib, changed course, saying he was willing to talk to the regime if it would help end bloodshed.

He suggested that Assad release tens of thousands of political prisoners as a first step.

Members of the opposition criticized al-Khatib's offer to talk to the regime, and the government flatly rejected it.

Late Friday, al-Zoubi said Damascus was ready for dialogue with the opposition so long as the rebels lay down their weapons. He said anyone who responds will not be harmed.

The initiative is unlikely to gain any traction among the Syrian opposition and fighters on the ground, a highly decentralized force with weak links to the political leaders that deeply distrusts the regime. Most groups are unlikely to stop fighting so long as Assad remains president.

In Cairo, al-Khatib met with international peace envoy to Syria, Lakhdar Brahimi, to discuss the opposition leader's initiative for talks with the Assad regime, according to a U.N. statement. The statement said the envoy "reiterated his support for (al-Khatib's) initiative and encouraged the coalition to continue in this direction."

Brahimi's efforts to stop the fighting in Syria have failed so far, leaving the international community at a loss for ways to end the civil war.

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Associated Press writer Zeina Karam and Aya Batrawy in Cairo contributed to this report.