Panos Moumtzis of U.N. OCHA said military escalation could make Idlib's situation "much more complicated and brutal" than other conflict zones in Syria.
Over the past years, tens of thousands of Syrians have been displaced from other parts of Syria into Idlib that borders Turkey. The latest waves of opposition fighters and their families came from eastern suburbs of Damascus known as eastern Ghouta, northern parts of Homs province as well as southern neighborhoods of the capital that were recently taken by government forces and their allies.
Moumtzis' comments came three days after an airstrike on the northwestern village of Zardana killed more than 40 people in one of the deadliest attacks in Idlib province this year.
On Sunday, activists and a war monitor said at least 10 people were killed in airstrikes in Taftanaz, also in Idlib, while five others were killed in the nearby towns of Binnish, Ariha and Ram Hemdan.
The U.N.'s children's agency, UNICEF, said the attacks on Idlib over the past days have killed 13 children. It added that one million children live in Idlib.
"These are deeply sad reminders that the war on children in Syria is far from over," said Geert Cappelaere, UNICEF's Regional Director in the Middle East and North Africa. "The fundamental principle of protecting children everywhere and at all times is still a far-fetched dream for far too many families."
Moumtzis said that 920,000 people were displaced within Syria between January and April, the highest number of any four-month span since the war began.
"With this escalation, with this deterioration, we worry about seeing really two and a half million people becoming displaced more and more toward the border of Turkey if this is to continue," Moumtzis said.
"There is no other location to further move them," he said adding that also in terms of the humanitarian assessment "we see that more than 80 percent of the population of Idlib really is in an extremely vulnerable situation."
OCHA is on "high alert" for possible further displacement among the estimated two and a half million people in Idlib, adding "There is no place left to go" within Syria.
Wouter Schaap, the Syria director of CARE, a leading humanitarian organization fighting global poverty, said the figure 920,000 of people displaced "is shocking, and shows that the war in Syria is far from being over. People are still fleeing bombings and shelling in bigger numbers than ever, and seeking protection and assistance far from home."
Schaap said CARE and its partners inside Syria have seen it firsthand this year, when tens of thousands of civilians had to leave their homes in eastern Ghouta and northern parts of Homs province "to find refuge in overcrowded shelters."
In eastern Syria, Syrian troops backed by Russian air forces repelled a militant attack on the historic town of Palmyra, according to state news agency SANA. The agency did not say who carried out the attack but the Islamic State group has presence nearby and in the past captured the town twice and destroyed some of its invaluable archaeological sites.
Associated Press writer Bassem Mroue in Beirut contributed to this report.
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