The United States Secretary of State John Kerry warned Syria stating that it would be hard to hold the country together if the fighting does not stop, as Syrian regime leader Bashar al Assad and opposition groups accepted a plan for cessation of hostilities to start on Saturday.
With hostilities reported on several fronts, the opposition backed by Saudi Arabia expressed doubts about the proposal, which excludes attacks by the regime forces and its Russian backers on DAESH and Al Qaeda-linked Nusra Front.
Saudi-backed opposition said that Russia had stepped up its air strikes since the plan was announced on Monday.
The Assad regime in Damascus stated that continued foreign help for the opposition could wreck the deal.
Kerry said that the US will soon know if the plan would take hold.
"The proof will be in the actions that come in the next days," Kerry told the Senate Foreign Relations Committee in Washington.
Kerry also said that if a political transition to a government to replace the present administration does not unfold in Syria there are 'Plan B' options, in a reference to undefined contingency plans believed to include military action.
The next month or two would show if transition process is serious and Assad would have to make "some real decisions about the formation of a transitional governance process that's real. If there isn't ... there are certainly Plan B options being considered," Kerry said.
"It may be too late to keep it as a whole Syria if we wait much longer," he added.
The plan for cessation of hostilities is the result of intense diplomacy to end the five-year-long war that has killed 250,000 and displaced millions helping to lead a refugee crisis in Europe.
However, the opposition said that the exclusion of DAESH and Nusra Front would give the regime a pretext to keep attacking them because its forces are widely spread in opposition-held areas.
The Syrian regime, backed by Russian air strikes since September, said it would coordinate with Russia to define which groups and areas would be included in what it called a "halt to combat operations."
The terminology reflects the difficulties of getting peace efforts under way, with talks in Geneva making no headway and the failure to end further fighting after a cessation of hostilities was announced on Feb. 12.
The United Nations describes the cessation as something that would precede a more formal ceasefire it is hoping to establish at some future date.
"A ceasefire implies a whole mechanism and agreements, signed agreements between the parties etc. This is a cessation of hostilities that we hope will take force very quickly and provide a breathing space for the intra-Syrian talks to resume," UN Spokesman Ahmad Fawzi said.
Assad opposes the word "ceasefire," saying it is something concluded between armies or states. Instead he has offered a "halt to combat operations."
"It does not happen between a state and terrorists," Assad said last week.
The Russian intervention in the fighting has turned the momentum Assad's way in a conflict that has mostly reduced his area of control to the big cities in the west and the coast.