Last week, the United Nations warned that such an offensive would likely lead to the "worst humanitarian catastrophe of the 21st century".
But while Erdogan had also called for a truce, Putin said this would be pointless as it would not involve the Islamist militant groups that Russian Federation deems terrorists, and Rouhani said Syria must regain control over all its territory.
"This would unleash a humanitarian nightmare unlike any seen in the blood-soaked Syrian conflict".
Burt said the United Kingdom officials have held contacts with Turkish counterparts on the continued efforts, adding that "it is deeply disappointing that Russian Federation and Iran rejected President [Recep Tayyip] Erdogan's calls for a renewed cease-fire" at the Tehran Summit last Friday.
Guterres said he was appealing to Iran, Russia and Turkey to "spare no effort to find solutions that protect civilians" and warned that any use of chemical weapons would be "totally unacceptable".
Russia's ambassador to the UN met Haley's threat with a denial that Syria has any chemical weapons, as the UN secretary-general warned of a potential "bloodbath" during Tuesday's UN Security Council meeting on the crisis in Idlib, Syria.
"A regime assault would also create serious humanitarian and security risks for Turkey, the rest of Europe and beyond", he would add.
A 20-truck convoy carrying humanitarian aid for Syria's northwestern province of Idlib set off from Istanbul on Monday, president of a Turkish aid agency said.
Nebenzia, the Russian diplomat, also summed up last week's summit among Russia, Turkey and Iran as "a major milestone in restoring peace and ensuring lasting settlement in Syria".
There is an urgent need to make greater progress in the Geneva process, and in particular the creation of a constitutional committee as part of the overall political package, he said. "He's been warned and so we'll see if he's wised up", Mattis added.
Idlib is the insurgents' only remaining major stronghold and a government offensive could be the war's last decisive battle.
Previously, the Russian Ministry of Defense had accused the USA government of planning a new round of airstrikes on Syria, using the false-flag attack allegedly being coordinated by terrorist groups on the ground in Syria as a pretext and justification.
She also said that "Russia has the power to stop the catastrophe looming in Idlib".
Vassily Nebenzia was briefing the council on last Friday's summit of the presidents of the three Astana countries in Tehran, stressing that they are committed "to continue helping with a definitive elimination of terrorism in Syria". "That's something we're monitoring very closely", OCHA spokesman David Swanson told reporters.
"Since September 4, an uptick in violence has killed scores of civilians and displaced more than 30,000 people, Laerke said".