More than 30,000 people have so far fled their homes in northwest Syria since Syrian government and allied forces resumed air and ground bombardments there last week, the U.N. agency coordinating relief efforts said on Monday.

The U.N. Office for the Coordination of Humanitarian Affairs (OCHA) said an all-out military assault on the last major stronghold of active opposition to President Bashar al-Assad could set 800,000 people to flight. The OCHA chief, Mark Lowcock, warned that this risked provoking the worst humanitarian catastrophe of the 21st Century.

Damascus, backed by Russia and Iran, has been preparing a major assault to recover Idlib and adjacent areas of northwest Syria from rebels.

Russian and Syrian warplanes resumed their bombing campaign last week and the presidents of Turkey, Iran and Russia on Friday failed to agree on a ceasefire that would forestall the offensive.

OCHA spokesman David Swanson told Reuters that as of Sunday, 30,542 people had been displaced from northwest Syria, moving to different areas across Idlib.

About 2.9 million people live in the opposition-held area, which comprises most of Idlib province and adjacent small parts of Latakia, Hama and Aleppo provinces. Around half of them are already displaced from other parts of Syria.

ON THE MOVE AGAIN

"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.

"There need to be ways of dealing with this problem that don't turn the next few months in Idlib into the worst humanitarian catastrophe with the biggest loss of life of the 21st Century," he said.

Swanson said that since Friday's summit, mortar and rocket attacks had increased, especially in the northern Hama countryside and southern Idlib rural areas.

He said 47 percent of those displaced have moved to camps, 29 percent are staying with families, 14 percent have settled in informal camps and 10 percent are in rented accommodation.

Abu al-Baraa al-Hamawi, a rebel leader in northern Hama, said about 95 percent of people had left a number of villages in northern and western Hama province and in southern Idlib province in the last three days due to intensive air strikes.

More than half a million people have been killed and 11 million already forced to flee their homes in Syria's seven-year-old war.

Christy Delafield of Mercy Corps, one of the largest organisations delivering aid in Syria, said it has been hard for aid workers and communities to keep up with the displacement.

"There is a lack of water storage capacity in many of the areas in which we operate, with just two or three days worth of water available to civilians," she told Reuters.

"The crossing points along the front lines between the government and opposition-controlled areas have been closed, and as a result, food prices have dramatically increased."

The opposition accuses Russia and its allies of striking at hospitals and civil defence centres to force rebels to surrender in a repeat of earlier, large-scale military offensives.

Russia 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.

The Syrian Observatory for Human Rights war monitor said rebel shelling on Monday had hit Hama military airport and another nearby military complex which lie in government-held territory.

U.N. Special Envoy Staffan de Mistura began two days of talks in Geneva on Monday with senior officials from Russia, Iran and Turkey on forming a Constitutional Committee in Syria, but which were expected to be overshadowed by the Idlib crisis.

Tehran and Moscow have helped Assad turn the course of the war against an array of opponents ranging from Western-backed rebels to Islamist militants. Turkey is a leading opposition supporter which has troops in the country and has erected 12 observation posts around rebel-held Idlib.

Turkish Defence Minister Hulusi Akar on Monday was reported as saying by broadcaster NTV that air and ground attacks on Idlib must stop and a ceasefire must be established.