Secretary-General Antonio Guterres is appealing to Russia and Turkey to stabilize the situation in Syria's last rebel-held territory in the Idlib province area, where Bashar Assad's forces have launched an offensive "without delay."

The U.N. chief expressed deep concern Tuesday at the escalation in fighting, calling the situation "especially dangerous given the involvement of an increased number of actors."

Turkey and Russia negotiated a cease-fire in Idlib last September but it has collapsed following the Syrian government offensive launched in late April.

Guterres told reporters at U.N. headquarters that civilians are again "paying a horrific price."

"As I have said repeatedly, there is no military solution to the Syrian crisis," he said.

The U.N. Security Council is scheduled to be briefed on the situation in Idlib later Tuesday.