Syria was also on the agenda at the UN Security Council on Tuesday, when Turkish Ambassador Feridun Sinirlioglu said "There is no doubt that an all-out military operation would result in a major humanitarian catastrophe".
United Nations Secretary-General Antonio Guterres on Tuesday appealed to Russia, Iran and Turkey to "spare no effort to find solutions that protect civilians" in Syria's Idlib and said it was "absolutely essential" a full-scale battle was avoided.
A trilateral summit of the presidents of Turkey, Russia and Iran last week underlined that "there could be no military solution to the Syrian conflict" and called for advancing the political process to reach a negotiated solution.
"We're deeply concerned about this recent escalation of violence, which has resulted in the displacement of over 30,000 in the area".
"With all our generosity, we have gone to extraordinary lengths to alleviate suffering of the Syrian people, hosting some 3.5 million refugees, more than any other country in the world".
"We are very actively preparing for the possibility that civilians move in huge numbers in multiple directions", OCHA head Lowcock told a news briefing in Geneva.
The forces of President Bashar Assad used the banned chemical weapons in attacks on Douma, near Damascus, on January 22 and February 1, as well as in the northern Idlib province on February 4, the report quoted by Reuters said.
Some three million people live in the zone now, about half of them already displaced by the brutal seven-year war and others heavily dependent on humanitarian aid to survive. "Why do we have to kill so many civilians?"
"The hypothesis of war crimes cannot be excluded. once one begins to indiscriminately bomb civilian populations and hospitals", Jean-Yves Le Drian told lawmakers.
Russian Federation has said it wants all militants to be pushed out of Idlib and that it avoids civilians and targets only radical al Qaeda-inspired groups.
US Ambassador to the United Nations Nikki Haley warned Russian Federation and Iran of "dire consequences" if they continue airstrikes against the last rebel-held area in Syria and said the US would respond to any use of chemical weapons. The talks are hosted by United Nations envoy Staffan de Mistura and also include senior Russian and Turkish officials.
Tehran and Moscow have helped Assad turn the course of the war against an array of opponents ranging from Western-backed rebels to the Islamist militants, while Turkey is a leading opposition supporter and also has troops in the country.