Secretary of State John Kerry told lawmakers he won't vouch for the success of the newly struck cease-fire agreement in Syria but said the truce negotiated with Russia is the best pathway for ending five years of violence that has killed more than 250,000 people and displaced another 11 million from their homes.

"I'm not going to say this process is sure to work because I don't know," Kerry said Tuesday during testimony before the Senate Foreign Relations Committee.

Kerry's appearance came just a day after Washington and Moscow announced the cease-fire, which takes effect Saturday, even as major questions loomed over enforcement of that truce and how violations of the agreement will be handled.

A deep mistrust of Russia was evident throughout the hearing, as Republicans and Democrats both expressed concern Moscow would not abide by terms of the cease-fire and press on with an air campaign that has strengthened Syrian President Bashar Assad's grip on power. Russia insists it is targeting terrorists, but the U.S. and its partners say the strikes are mainly killing moderate rebels and civilians.

Sen. Barbara Boxer, D-Calif., expressed concern the truce will become a "rope-a-dope deal," a reference to boxing technique in which a fighter exhausts his opponent while expending little energy.

"It may be," Kerry acknowledged. "So it's step by step. No illusions. Eyes are open."

But he stressed there can be no political resolution to the conflict without a cease-fire as a starting point. And if the truce leads to the flow of humanitarian relief into Syria and lives are saved, "then that's a benefit," he said. But, Kerry added, "it doesn't mean that's it automatically going to have a positive outcome on the political process."

The truce will not cover the Islamic State extremist group, the al-Qaida-affiliated Nusra Front and any other militias designated as terrorist organizations by the U.N. Security Council. Both the U.S. and Russia are still targeting those groups with airstrikes.

A similar cessation announced Feb. 11 and set to begin last week did not take place, while Russian airstrikes continued to support a Syrian government offensive in the northern province of Aleppo.

Sen. Rand Paul, R-Ky., suggested backing away from the demand that Assad step down as a precondition. "Sure he's a terrible guy," Paul said of Assad. "The Middle East is full of them. Half the countries over there have despots."

But Kerry said Assad has to go.

"As long as Assad is there, you cannot stop the war," Kerry said. "People don't see how someone who has gassed his own (citizens), driven so many of them into refugee status, tortured them, starved them, bombed them, how he's somehow going to be the glue that brings the place together is beyond anybody's understanding."

Kerry sparred with Republicans on the panel over whether President Barack Obama has the leverage and the will to punish Moscow if it doesn't live up to the terms of the cease-fire.

"Russia knows there will be no Plan B," said Sen. Bob Corker, R-Tenn., the committee's chairman. Sen. John Barrasso, R-Wy., said there is little incentive for Russia to change course. "To me the only thing Russia has been consistent about is failing to keep its word," Barrasso said.

Kerry said there discussions under way within the Obama administration for a backup strategy "in the event we don't succeed at the (negotiating) table." He also said it would be a mistake to underestimate Obama's potential for taking punitive action against Moscow. He declined to say what those moves would be.

"This is a moment to try to see whether or not we can make this work," Kerry said, "not to find ways to preordain its failure and start talking about all the downsides of what we might do afterward."

Syria's government on Tuesday accepted the cease-fire, reserving the right to respond to any violations of the truce. The main opposition and rebel umbrella group also approved the deal but set its own conditions for compliance.

But exactly where along Syria's complicated front lines the fighting would stop and where counterterrorism operations could continue under the truce is still to be addressed.

Syria's Foreign Ministry said it accepts the proposed truce but that its operations will continue against IS, al-Qaida's branch in Syria and "other terrorist groups." It also stresses the right of its armed forces "to retaliate against any violation carried out by these groups."

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