Though rockets hit both Syrian government- and rebel-held territory on Friday morning, the Syrian Observatory for Human Rights said the level of violence was significantly reduced.

"Yesterday, for example, there were already air strikes by early morning," Observatory Director Rami Abdulrahman said.

More than 400 civilians have been killed in the escalation over the past three months and more than 440,000 displaced, the U.N. Office for the Coordination of Humanitarian Affairs said last week.

Syrian state media reported that a ceasefire would apply from Thursday night on condition militants comply with an agreement between Russia and Turkey last year which aimed to create a demilitarised buffer zone.

There has been no official comment from rebels in the area, which is dominated by jihadist factions but where Turkey-backed opposition groups also operate. The Turkey-backed opposition is taking part in Syria talks being hosted by Kazakhstan.

A Kazakh Foreign Ministry official said most of the opposition delegations at the talks had agreed to a ceasefire, Russia's RIA news agency said.

"A ceasefire has entered force, as the participants have said," it cited the official as saying. But the official noted that the agreement did not include jihadists who might not observe it.

The areas targeted by the government side are part of the last major piece of territory held by rebels who have been defeated across much of Syria by President Bashar al-Assad and his allies Russia and Iran.

But Assad's side has been unable to gain much ground in this latest offensive against the rebels. The government says it has been responding to rebel attacks.

In addition to the civilian casualties, the fighting has inflicted a heavy toll on both rebel and government fighters: the Syrian Observatory for Human Rights says each side has lost around 1,000 fighters.

The most powerful faction in the northwest is the jihadist Tahrir al-Sham group. Turkey has forces on the ground in the area at a dozen military positions.

The United Nations will investigate attacks on U.N.-supported facilities and other humanitarian sites in the area, Secretary-General Antonio Guterres said on Thursday, two days after two-thirds of the Security Council pushed for an inquiry.

Britain, France, the United States, Germany, Belgium, Peru, Poland, Kuwait, the Dominican Republic and Indonesia delivered a demarche - a formal diplomatic petition - to Guterres on Tuesday over the lack of an inquiry into attacks on some 14 locations.