Secretary of State John Kerry told lawmakers on Tuesday he is uncertain whether the proposed cease-fire in Syria will work, but said it is the only option on the table if a political settlement in the war-torn country is the goal.

Still, Kerry told the Senate Foreign Relations Committee, he’s “not going to vouch” for the plan. 

“I’m not going to say this process is sure to work because I don’t know,” he said.

Kerry’s comments come one day after the U.S. and Russia pitched a plan to implement a cease-fire in Syria. Despite the deal, nagging questions remain over the enforcement of the truce and how violations of the agreement will be handled.

While skeptical of its success, Kerry told lawmakers that if the cease-fire, which goes into effect Saturday, leads to the flow of humanitarian assistance to the ravaged country, it would be “a benefit.”

Sen. Barbara Boxer, D-Calif., said she's concerned Russia won't honor the truce and it will become a "rope-a-dope deal."

"It may be," Kerry replied.

Sen. Bob Corker, R-Tenn., the committee's chairman, said he has no confidence Russia would abide by the cease-fire agreement. Corker also said Russia is using refugees as a "weapon of war" against Europe. Corker and other Republican senators chided Kerry for the lack of leverage the U.S. has against Russia if Moscow violates the terms of the agreement.

"Russia knows there will be no Plan B," Corker said. But Kerry said it would be a mistake to underestimate President Obama's potential for taking punitive action against Moscow.

The war in Syria has claimed the lives of more than 250,000 people and displaced 11 million more.

The truce, which is set to go into effect Saturday, does not cover the Islamic State extremist group, the Al Qaeda-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.

The Syrian government and the main umbrella for Syrian opposition and rebel groups announced Tuesday their conditional acceptance of the cease-fire.

But even if the cease-fire is implemented, the fighting and violence in Syria won't stop.

Despite the agreement, Russia is almost certain to continue an air campaign that it insists is targeting terrorists. But the U.S. and its partners said Russia is mainly hitting moderate opposition groups and killing civilians.

While ISIS tries to expand its self-proclaimed caliphate in Syria and neighboring Iraq, al-Nusra is unlikely to end its effort to overthrow Syrian President Bashar Assad. The Kurds have been fighting ISIS, even as they face attacks from America's NATO ally Turkey. And Assad has his own history of broken promises when it comes to military action.

In congressional testimony two weeks ago, Director of National Intelligence James Clapper said Russia's campaign in Syria "has eclipsed its aggression in the Crimea and Ukraine as the most serious flashpoint in U.S.-Russian relations."

"Unlike Russia's obscured hand in Ukraine, its actions in Syria are being played out in daily headlines that report on Russia's indiscriminate bombing and its support of the Syrian regime in areas where moderate forces are aiming to get out from under the rule of the Assad regime," Clapper said.

As objectionable as Russia's involvement in Syria is, the only prospect for peace is through a negotiated cease-fire, humanitarian relief and a serious attempt at negotiating a political resolution, said Sen. Chris Coons, D-Del. That approach is "far preferable to our deploying tens of thousands of American troops in an attempt to move the balance of the battlefield back against Assad," he said.

The Associated Press contributed to this report.