The top Republican on the Senate Foreign Relations Committee urged Secretary of State John Kerry Thursday to quickly move forward with a "Plan B" after Syria peace talks collapsed in Geneva Wednesday.

"Now that the cessation of hostilities is effectively dead, it looks like it's time to consider what Secretary Kerry referred to as a 'Plan B,' and I look forward to him laying that out very soon," Sen. Bob Corker, R-Tenn., said in a statement Thursday. He was referring to the formal ceasefire agreement between the Assad regime, the opposition and other parties hashed out in February.

Corker said he spoke to the head of Syria's main opposition group earlier Thursday and understands why it decided to pull out of formal participation in the Geneva-based talks, after the agreed ceasefire crumbled in recent weeks and violence once again became the norm in areas of the war-torn country.

"The Assad regime continues to target civilians, block humanitarian access, and refuse the release of detainees," Corker said. "Meanwhile, Russia and Iran continue to flood weapons, heavy equipment and personnel into Syria as violence is picking up throughout the country."

Corker spoke to Riyad Hijab, a senior representative of the Syrian opposition who heads the U.S. and Saudi-backed High Negotiations Committee, about his decision to leave the Geneva talks Wednesday amid the mounting violence. The HNC suspended its involvement in the talks Tuesday in response to the ongoing conflict that has cut off humanitarian access to several parts of the country.

Back in early March, Kerry made several references to a "Plan B" during testimony to the Senate Foreign Relations Committee, what some in Congress and the diplomatic community interpreted as a stepped-up U.S. military campaign to help the Syrian opposition's efforts to overthrow Assad.

"This could get a lot uglier," Kerry warned at the time.

President Obama, after a summit with Gulf leaders during his visit to Saudi Arabia on Thursday, said the Syrian ceasefire had come under "enormous strain" and said the U.S. and its allies in the region would continue to work for a "political transition away from" Assad.

At the conclusion of the summit, deputy national security adviser Ben Rhodes said the ceasefire over the last several weeks had "provided an opening" for relative calm in some areas of the country and the opportunity to distribute humanitarian relief. But, he said, there have been "persistent violations" of the ceasefire from the Assad regime in Northwest Syria, and the al Nusra terrorist group.

"It's important now that the U.S., our Gulf partners are reinforcing the importance of this opportunity, and we're strongly urging Russia to use its influence, and frankly Iran as well, to sustain what has been an opening" for the Syrian people to experience a break from the violence.