UNITED NATIONS (AP) — The U.N. director of humanitarian operations warned Tuesday that a large-scale Syrian government offensive in the last major rebel stronghold in northern Idlib province "has the potential to create a humanitarian emergency at a scale not yet seen" in the seven-year civil war.

John Ging called on members of the U.N. Security Council Tuesday "to do all they can to ensure that we avoid this."

The Syrian government is gearing up for an expected offensive in Idlib province, which is home to nearly 3 million people and has a large al-Qaida presence in addition to Syrian rebel groups. It borders Turkey, which fears an offensive may trigger a humanitarian and security catastrophe.

Ging said recent weeks have seen "a further serious deterioration of the humanitarian situation" with intense bombing and shelling reported in parts of Idlib as well as Aleppo, Hama and Latakia provinces resulting in civilian deaths and destruction of schools and hospitals.

He said humanitarian organizations are trying to sustain their current response and prepare for a possible further deterioration, "but a worst-case scenario in Idlib will overwhelm capacities and has the potential to create a humanitarian emergency at a scale not yet seen through this crisis."

The United States, Britain and France have warned that they will respond "appropriately" to any chemical weapons attack in Idlib — a warning repeated to council members Tuesday by all three countries.

Russia again accused Syrian rebels of preparing a chemical attack, which Moscow says the West will use to justify a strike against Syrian forces. Russia also claims British special forces are helping the rebels, including by possibly supplying chlorine, allegations vehemently denied by Britain's U.N. Ambassador Karen Pierce who called them "baseless" and "outlandish."

"Having heard their case," Pierce told reporters after the council meeting, "it's very clear to us and our partners that this is an effort to distract from an imminent attack on Idlib."

Russia's U.N. Ambassador Vassily Nebenzia said Syrian armed forces have no chemical weapons and there is no military need for them.

"People in their right minds will not use means that are useless from a military point of view in order to trigger reprisals by three major powers. But of course publicity about what is expected provocation could unleash the strikes," he said.

"Thank you for this warning of your response. We want to also warn you that we are absolutely aware of your unsavory plans," Nebenzia said. "We strongly suggest that you refrain from them. Carrying out these plans will further deliver a strike against a peaceful resolution in Syria."

Pierce, the current Security Council president, was asked whether in closed consultations she saw any prospect that the 15 members would unite and respond to Ging's plea to avoid a humanitarian disaster in Idlib.

"The short answer is no, and that's very sad," she said. "In closed consultations the Russians backed off their original consultations a little bit. I'd like to think that that meant we might be able to get to a sensible outcome on Idlib in the council that helps protect civilians."

"But I'm afraid all the evidence so far is against us. We will still try," Pierce said.