"The greatest immediate threat to the people of Ukraine realizing a better future is the external threat of Russia," Sen. Bob Corker of Tennessee said during a visit to Ukraine.
"When I return to Washington, I will continue urging the Obama administration to push back on Russia in a much more forceful way by imposing sanctions against portions of the banking and energy sectors ... to deter further Russia aggression."
Corker said the U.S. should target Russian energy giant Gazprom, one of the world's largest companies.
The lawmaker also expressed support for Ukraine's scheduled national elections next month.
"It is critical that the scheduled May 25 election occur, that it be fair and free," he said. "I know that is a priority of [the Ukrainian] government and some of the leading candidates that I met with today."
Corker said Eastern Europe's stability hinges on Ukraine.
"The stakes are very high. This situation is not just about Ukraine; it's also about other vulnerable places like Moldova and ultimately - if not addressed - other countries in Europe and the former Soviet Union," he said.
"It is crucial that we act now to prevent Russia's perverse actions from gaining momentum and affecting other countries in the region."
The U.S. also has authorized a $1 billion loan guarantee for Ukraine's fledgling government.
Obama says his administration is prepared to take further action against Russia if diplomatic efforts to ease the conflict fail.
Corker is on a four-day visit to Moldova and Ukraine to assess the situation in the region since Russia's controversial annexation of Crimea and the recent takeover of government and police buildings in eastern Ukraine by pro-Russian forces.