"The stakes are high" for millions of people in Idlib, Lowcock said. He said a military offensive "would overwhelm all ability to respond" with humanitarian aid, "devastating a population that is already weakened through years of conflict, displacement and deprivation."

US deputy ambassador Jonathan Cohen said it was "chilling" that Syria's UN ambassador, Bashar Ja'afari, warned Friday that the government "will retake Idlib at a time of its choosing."

"What this means is that the window of opportunity to prevent the slaughter of millions of Syrian civilians living in Idlib, created by Turkey and Russia's agreement on a demilitarized zone, is closing," Cohen said. "And it can close fast."

Ja'afari did not repeat his warning Monday. Instead, he reaffirmed Syria's commitment to the Russia-Turkey agreement.

"The Syrian government will continue to facilitate all attempts to cease the spilling of blood," he said.  "We welcome all initiatives to bring peace and security to all of the Syrian territory that has been struck by terrorism."

But Ja'afari also reaffirmed that Syria's government "will continue to combat terrorism until it liberates the last part of Syrian territory from terrorists" and from all foreign military "illegally on Syrian territory."

Russian deputy ambassador Vladimir Safronkov, whose country backs Syria militarily, said the agreement on Idlib was "temporary" and noted there has been shelling of nearby towns and villages in violation of the truce's terms.

"Eliminating the terrorist threat in Syria has not been taken off the agenda by anyone," Safronkov said. "If provocations from radical elements persist, Russia reserves the right to support the effective actions of the Syrian government to eliminate hotbeds of terrorism." 

Lowcock, the UN undersecretary-general for humanitarian affairs, also called for an aid convoy to be given immediate access to the desolate Rukban area near Syria's border with Jordan, where up to 50,000 civilians haven't received food and other humanitarian assistance since January.

He said the UN and the Syrian Arab Red Crescent had prepared a delivery to Rukban on Saturday for 50,000 people and a vaccination campaign for 10,000 children, "but reports of insecurity along the route forced us to postpone the convoy."

Ja'afari cited unspecified reports that fighters from the Islamic State extremist group were going to attack the convoy "from the zone controlled by the Americans." He accused the U.S. of impeding the delivery of aid.

The Security Council's open session followed a meeting in Istanbul on Saturday of the presidents of Turkey, France and Russia and the German chancellor who expressed hope that the cease-fire in Idlib can provide some momentum for peace efforts. A statement from the leaders called for "an inclusive, Syrian-led and Syrian-owned political process," and the convening of a committee by the end of the year to draft a new constitution, which is key to holding UN-backed elections.

Russia's Safronkov told the council: "We expect that with the right conditions it will be able to begin its work by the end of this year."

But he stressed that imposing ultimatums and "extreme deadlines" won't work, and all parties must understand that for the committee "to be effective and viable the structure must be recognized as legitimate by all parties."

Agreement has been reached on 50-member delegations from the Syrian government and from the opposition for the drafting committee. But the government objects to the 50-member delegation that UN envoy Staffan de Mistura was authorized to put together representing Syrian experts, civil society, independents, tribal leaders and women.

De Mistura, who is stepping down as Syria envoy at the end of November, attended Saturday's meeting in Istanbul and followed up Monday, holding talks in London with senior officials from Egypt, France, Germany, Jordan, Saudi Arabia, the United Kingdom and United States. His office said that "it was a useful interaction based on the seriousness and urgency of moving ahead with the constitutional committee."