UN Secretary-General Antonio Guterres issued a new call for "an immediate cessation" to military operations in southwest Syria, where government forces are attacking rebel-held areas.

Guterres is "deeply alarmed by the military offensive in southwestern Syria and its devastating toll on civilians," said a statement from his spokesman Stephane Dujarric said yesterday.

"The secretary-general recalls that the southwest area of Syria is part of a de-escalation agreement agreed between Jordan, Russia and the United States," the statement said.

Guterres "calls on all parties to respect their obligations under international humanitarian law and human rights law, protect civilians and facilitate safe, unimpeded and sustained humanitarian access." The UN chief had made a similar call earlier this month, after Russian-backed government forces began attacking opposition-held parts of Daraa province on June 19.

Tens of thousands of Syrians have fled since the beginning of the offensive, and the Syrian Observatory for Human Rights monitoring group says that nearly 100 civilians have been killed.