NEW YORK — As Syria’s seven-year civil war drags on, no medical or food supplies have reached besieged populations in the country since November and “humanitarian diplomacy seems to be totally impotent,” according to a top U.N. official.

Amid renewed fighting between Turkey and U.S.-allied Kurdish militias in northern Syria, United Nations Special Adviser Jan Egeland said some 2.9 million people are estimated to have been displaced in 2017, at a rate of almost 240,000 per month, and he called on Russia, Turkey and Iran to “de-escalate” the crisis. “We are getting nowhere at the moment,” Egeland said Thursday in Geneva. “The situation is screaming for a cease-fire.”

Separately, U.S. officials said Russia is complicit with Syrian leader Bashar Assad’s continuing use of chemical weapons despite a 2013 agreement to give up chemical stockpiles and not use weapons banned by international treaties. Assad and Russia have denied that such weapons have being used.

The officials said Assad’s use of chemical weapons, including sarin and chlorine, is intended to both stoke fear among his people and compensate for a lack of sufficient troops and conventional weapons, despite receiving military backing from Russia since 2015.

Assad’s regime is trying to permanently alter Syria’s demographics, limiting the return of refugees and Sunni Muslim populations in particular, according to two U.S. administration officials who spoke to reporters on condition of anonymity. The U.S. fired missiles at Syrian targets last year, in what Washington said was a response to a chemical attack, and the officials signaled that similar action could be taken again.

In the latest turn of events on the ground, Syrian troops are battling rebel forces in the besieged city of Idlib, the largest opposition stronghold, just as Turkish troops are fighting U.S.-backed Kurdish forces in northern Syria.