Government forces shelled Syria's last major rebel bastion on Wednesday, hours after UN chief Antonio Guterres warned the Security Council any full-blown offensive in Idlib risks triggering a "bloodbath".
As troops massed for a Russian-backed offensive in the northwest, Kurdish-led rebels launched a US-backed assault in the east to oust the Islamic State (IS) group from its last redoubt in the Euphrates Valley, the US-led coalition confirmed.
Intermittent artillery fire hit southern districts of Idlib province and adjacent rebel-held areas of Hama province, the Syrian Observatory for Human Rights said.
The Britain-based war monitor did not immediately report any casualties from the bombardment which came after shelling and air strikes killed at least 15 civilians in the rebel zone since September 4.
The northwestern province and adjacent areas form the largest chunk of territory still held by the rebels, who have been worn down by a succession of defeats in other parts of the country.
Russia-backed government forces have been massing for weeks around Idlib, which is home to nearly three-million people, many of them already dependent on aid.
UN agencies and relief organisations have warned repeatedly that any major assault could spark one of the worst humanitarian disasters of Syria's war.
"It is absolutely essential to avoid a full-scale battle in Idlib," Guterres said on Tuesday.
"This would unleash a humanitarian nightmare unlike any seen in the blood-soaked Syrian conflict."