GENEVA — The United Nations expressed alarm on Thursday about a surge of fighting and destruction in the Idlib Province of northwestern Syria, the last major area of the country held by insurgents, where assaults by Russian-backed Syrian forces have put tens of thousands of civilians at risk.

United Nations relief officials also called on Thursday for an urgent humanitarian pause in fighting around Eastern Ghouta, the rebel-held Damascus suburb where roughly 400,000 civilians have long been trapped without access to emergency aid.

The assault on Idlib, including areas near the Turkish border, has forced more than 100,000 people to flee for safety since the start of December, Jan Egeland, the United Nations adviser on humanitarian affairs in Syria, said after a meeting of a humanitarian task force in Geneva on Thursday.

The United Nations estimates Idlib’s population at 2.5 million, including more than a million who fled or were evacuated to the province to escape offensives elsewhere in the country, and who are packed into camps scattered across the province.