The United States warned Syria and its allies Russia and Iran that if they assault Idlib, the last major stronghold for Syrian rebels in Idlib, "the consequences will be dire."
U.S. ambassador to the UN Nikki Haley made the remarks to the UN Security Council on Friday.
Eight aid agencies also called on world leaders to prevent a humanitarian catastrophe in Syria's northwestern Idlib province.
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The agencies called on leaders of the Iran, Russia and Turkey meeting in Tehran and members of the UN Security Council meeting later in New York to work together to find a diplomatic solution that can protect civilians, aid workers and allow access to humanitarian agencies to the overcrowded province and surrounding areas.
More than 3 million live in Idlib and its environs, many of them already displaced by conflict elsewhere in Syria.
The agencies, including CARE, Mercy Corps and the International Rescue Committee, said the "most vulnerable" are likely to pay the heaviest price in case of an offensive. They said aid workers working in Idlib are already overwhelmed trying to provide basic needs and shelter to the province's population, which has doubled in size in recent months because it is hosting displaced citizens from all over Syria.
Turkey's president called for a cease-fire Friday at a trilateral meeting between Iran, Russia and Turkey in Tehran. He called for an end to airstrikes amid the looming campaign for Syria's Idlib province. Recep Tayyip Erdogan said his country does not have the "strength or capability" to host millions of more refugees from Idlib.
Erdogan also said Friday that a call for terror groups in Idlib to lay down arms was a strong message to them and would help halt refugee flows.
"We have to take joint steps to prevent the migration, we need to be successful in the fight against terrorism," Erdogan said.
"Turkey is already sheltering 3.5 million refugees (from Syria). The population of Idlib is 3.5 million. Turkey doesn't have the strength or capability to host 3.5 million more," he added.
Meanwhile, Iranian President Hassan Rohani said "we have to force the United States to leave" Syria. He also said the fight in Syria should continue until all extremists are "uprooted," especially in Idlib.
Residents in Idlib held mass rallies in the rebels' last bastion, protesting an imminent government offensive there and chanting against the country's ruler President Bashar Assad.