The United Nations’ humanitarian chief on Wednesday said a Russian-backed Syrian offensive has forced thousands to flee into jampacked areas near the Turkish border “under horrendous conditions” that are killing babies and young children.

Mark Lowcock told the U.N. Security Council that "the unfolding humanitarian catastrophe" in northwest Idlib province, which is the last major rebel stronghold, has "overwhelmed" efforts to provide aid.

He said nearly 900,000 people – more than half of them children – have been displaced since Dec. 1 when the government offensive began.

"I am getting daily reports of babies and other young children dying in the cold," he added.

U.N. special envoy Geir Pedersen echoed Secretary-General Antonio Guterres' expression of alarm on Tuesday at the rapid deterioration of the humanitarian situation "and the tragic suffering of civilians."

"Hostilities are now approaching densely populated areas such as Idlib city and Bab al-Hawa border crossing, which has among the highest concentration of displaced civilians in northwest Syria and also serves as a humanitarian lifeline," he said.

Pedersen said Russia and Turkey "can and must play a key role in finding a way to deescalate the situation now.”

TURKEY THREATENS 'IMMINENT' OPERATION IN IDLIB AFTER RUSSIAN-BACKED SYRIAN MILITARY PUSHES TOWARD BORDER

The United States, United Kingdom, Germany and others have stressed that three-way talks with Syria supporters Russia and Iran and opposition backer Turkey, which led to a de-escalation zone in Idlib, aren't working.

German Ambassador Christoph Heusgen said it's time for the U.N. to step in and "it's time also for the secretary-general also to step up to the plate."

U.S. Ambassador Kelly Craft told the council that "the clearest path we see to an immediate end to violence in northwest Syria is for the U.N. to take full charge of a new cease-fire initiative."

Russian Ambassador Vassily Nebenzia supported Pedersen's efforts to get agreement from Syria's government and opposition on an agenda so a constitutional committee can start discussing a new charter for the country, which is seen by many as a first step toward elections and formation of a new government.

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French Ambassador Nicolas De Riviere issued a statement this week calling for a cessation of hostilities in northwestern Syria, but Russia insisted on an additional line that would have allowed the fight against "terrorists" to continue. That was unacceptable to the vast majority of council members, the diplomats said, speaking on condition of anonymity because the consultations were private.

The Associated Press contributed to this report.