UN Security Council members had been due to meet in New York on Friday afternoon for a hastily called meeting on the fragile Syrian ceasefire, billed as the "last chance" to end the five-year war.

But Vitaly Churkin, Russia's ambassador to the United Nations, said the meeting was canceled at the last minute as the US was unwilling to disclose exactly what was in the documents outlining the deal hammered out last week by US Secretary of State John Kerry and Russian Foreign Minister Sergey Lavrov.

"This briefing is not going to happen and mostly likely we're not going to have a resolution of the Security Council because the US does not want to share those documents with the members of the Security Council and we believe that we cannot ask them to support a document which they haven't seen," Churkin said.

In Washington, a US official said the session was canceled because the Russians were trying to force the US to make the ceasefire deal public.

"The United States will not compromise operational security," the official said.

Ceasefire violations

Russia, a key backer of Syrian President Bashar al-Assad, said earlier Friday that Moscow was pushing for a 72-hour extension to the current ceasefire. The truce was implemented on Monday after the US and Russia reached an agreement two days earlier.

The ceasefire has been marred by violations throughout the week, however, including a lack of humanitarian aid deliveries and sporadic violence, which resulted on Friday in the deaths of three civilians, including two children.

In a meeting with US security aides, US President Barack Obama expressed his concern that despite the truce, the Syrian government continues to block the flow of humanitarian aid.

A statement from the White House said Obama "emphasized that the United States will not proceed with the next steps in the arrangement with Russia until we see seven continuous days of reduced violence and sustained humanitarian access."

A senior Russian military official said Moscow would help ensure the ceasefire in Syria for another three days, but warned the US to press the rebels to end violations of the truce.

ksb/cmk (AP, Reuters)