Kerry: US-Russia plan 'may be last chance to save Syria'

US Secretary of State John Kerry on Monday urged Syria's warring parties to observe a US-Russian brokered ceasefire and warned it could be the last chance to save the country.

Just hours after the truce came into effect, Kerry said it must hold in order to salvage any hope of the political settlement he said was the only way to end the war.

"For all the doubts that remain -? and there will be challenges in the days to come -? this plan has a chance to work," Kerry told reporters in Washington.

"We believe that the only realistic and possible solution to the conflict is ultimately a political outcome," he said, calling for a return to UN-mediated peace talks.

"And I urge all the parties to support it because it may be the last chance that one has to save a united Syria."

Kerry and his Russian counterpart Foreign Minister Sergei Lavrov announced Friday they had agreed on a plan to impose a cessation of hostilities across Syria, where more than 290,000 people have been killed since 2011.

Russia is to pressure its ally Syrian leader Bashar al-Assad to obey the truce, while the United States will urge the rebel groups it supports to refrain from attacks.

Two jihadist groups -- the Islamic State group and the Al-Qaeda affiliated Al Nusra Front -- are excluded from the truce and will continue to face air strikes.

Kerry said that "earliest reports" that he had received two hours after the truce came into effect at 1600 GMT suggested there had been "some reduction" in violence.

"There will undoubtedly be reports of a violation here or there, but that's the nature of the beginning of a ceasefire, almost always," he said.

Kerry said that the next stage if the truce takes hold would be for Assad's forces to allow the United Nations to ship humanitarian aid to the besieged city of Aleppo.

For this to happen, he said, the regime and Russian forces would have to allow passage through two routes that have recently been cut: the Castello Road and the Ramussa gap.

"Humanitarian assistance needs to begin to flow," he said.

"Now that can take a day or two or so, it depends, but the United Nations has indicated that they are prepared and preparing to take those deliveries in."