U.S. lawmakers have expressed skepticism about Russia’s commitment to a new cease-fire plan for Syria and plans for a political transition in the country.

In a Tuesday hearing on the State Department’s proposed budget, lawmakers sought assurances from Secretary of State John Kerry that Russia would curb its bombing campaign over Syria, which has focused largely on the Syrian regime’s opponents, not terrorists.

“[Russian leader Vladimir] Putin is attempting to change the battlefield dynamics to bolster the Assad regime and weaken the opposition in terms of anything related to peace,” Republican Senator John Barrasso of Wyoming said during the Foreign Relations Committee hearing.

He commented a day after the U.S. and Russia, the co-chairs of a cease-fire task force, announced plans to launch a partial cessation of hostilities in Syria on Saturday.

Kerry told lawmakers he was uncertain whether the cease-fire would work or lead to a negotiated peace settlement. But he said the plan was the best way to try to end the five-year conflict that has left as many as 470,000 dead, according to some estimates. It calls for a halt to fighting, except for continued attacks on Islamic State and al-Nusra Front fighters .

“If humanitarian assistance flows, if the guns do silence with the exception of the effort against Daesh and al-Nusra on Saturday, if they do, and lives are saved, then that is to the benefit," Kerry said. But he added, "It doesn’t mean that it’s automatically going to have a positive outcome in the political process."

Truckloads of aid

The secretary noted that in the last two weeks, 114 truckloads of humanitarian aid had been dispatched into areas of Syria that have been cut off by the fighting between Syrian government troops loyal to President Bashar al-Assad and rebel forces trying to overthrow him.

"At least 80,000 people who haven’t had supplies in years now have supplies for the next month at least, and we have resulted in [getting] food and medicine to places that have been under siege for months," Kerry said.

Some Senate lawmakers voiced skepticism that Russia would honor the truce, with Democratic Senator Barbara Boxer of California questioning whether the Washington-Moscow pact might turn out to be a "rope-a-dope deal" with frequent cease-fire violations.

Barrasso said Russia had been “consistent” in “failing to keep its word.” He asked Kerry what consequences the U.S. was prepared to impose upon Russia if it violated the cease-fire agreement.

“There is significant discussion taking place now about Plan B in the event we don’t succeed at the table,” said Kerry.

Committee Chairman Bob Corker, a Tennessee Republican, disagreed, saying, “I think the secretary is negotiating a situation where there is no Plan B. Russia knows there will be no Plan B” if it does not abide by terms of the agreement.

Support for Assad

There have been ongoing concerns about Russia’s support of Assad, even as Moscow continues to participate in the International Syria Support Group, which has agreed on a plan for a political transition in Syria.

Kerry said it would be a “mistake” to calculate that President Barack Obama would decide there were no further options if Russia backed away from commitments to a cease-fire.

Analysts say the U.S. may have little leverage to use against Russia for its actions in Syria, unless it considers military options.

“As things now stand, Russia is succeeding at driving back the Syrian opposition,” said Daniel Serwer, a professor at the Johns Hopkins School of Advanced International Studies.

“It has gained advantage on the battlefield. That’s not a particularly ripe situation for a political settlement,” added Serwer, who is also an analyst at the Middle East Institute.

Despite congressional doubt about Russia’s commitment to a cease-fire, the U.S. and other world powers are moving ahead with the cease-fire effort, which needs to be endorsed by both the Syrian government and the opposition by Friday.

“We urge the maximum number of armed opposition factions to express their support and readiness to participate in the cessation,” State Department spokesman John Kirby said Tuesday.

Tuesday’s testimony marked the first in a series of appearance on Capitol Hill this week for Kerry, as lawmakers consider the State Department’s proposed budget of $50.1 billion for fiscal 2017.

Email controversy

Senate Foreign Relations Committee members pressed Kerry on a number of issues, including the normalization of ties with Cuba and the implementation of the Iran nuclear agreement.

Political overtones factored into the hearing.

Senator Ron Johnson, a Wisconsin Republican, questioned Kerry about the ongoing State Department email probe involving his predecessor, former Secretary of State Hillary Clinton. Johnson said a Senate committee’s request for logs related to the transfer of classified information had gone unanswered for five months.

Kerry responded that he did not know the specific reason for the delay. He added that there were “more than 50 simultaneous investigations” underway regarding the emails and an “unprecedented” number of Freedom of Information requests.

Clinton is a Democratic contender in the U.S. presidential race.

VOA's Ken Bredemeier contributed to this report.