Kerry issues warning as Syrian parties back halt to fighting

With hostilities reported on several fronts, rebels backed by Saudi Arabia expressed doubts about the proposal, which excludes attacks by the Syrian army and its Russian backers on the jihadist groups Islamic State and the al Qaeda-linked Nusra Front. 

Saudi-backed rebels said Russia had stepped up air strikes since the plan was announced on February 22.

For its part, the government in Damascus has made clear that continued foreign help for the rebels could wreck the deal.

Secretary of State John Kerry said the United States would soon know if the plan would take hold. "The proof will be in the actions that come in the next days," he told the Senate Foreign Relations Committee in Washington.

If a political transition to a government to replace the current administration does not unfold in Syria, there are options, Kerry said, in a reference to undefined contingency plans believed to include military action.

Faced with skepticism about the cessation plan, Kerry said that things in Syria could get uglier.

"It may be too late to keep it as a whole Syria if we wait much longer," he said.

Kerry insisted Washington is working on ways to react if diplomacy does not work. "There is a significant discussion taking place now about Plan B if we don't succeed at the table," Kerry said.

France said the leaders of the United States, France, Britain and Germany hoped the cessation deal could take effect soon.

The plan is the result of intense diplomacy to end the five-year-long war that has killed 250,000 and forced millions to flee their homes helping to cause a refugee crisis in Europe.

But rebels say the exclusion of Islamic State and Nusra Front will give the government a pretext to keep attacking them because its fighters are widely spread in opposition-held areas.