His comments, made in a phone call with Iranian President Masoud Pezeshkian, came as the Syrian military and its ally Russia conducted deadly air raids on areas under Islamist and Turkish-backed militants control.

Assad is backed by Russia and Iran, both of which have confirmed they will help his army fight back after Aleppo, the country's second city, fell out of government control.

"The terrorist escalation reflects the far-reaching goals of dividing the region and fragmenting the countries in it and redrawing the map in line with the objectives of the United States and the West," Assad said in a statement from his office.

During the weekend, Al-Qaeda's former Syria branch "Hayat Tahrir al-Sham (HTS)" group and allied factions took over Aleppo city, barring neighbourhoods controlled by Kurdish forces, according to the Syrian Observatory for Human Rights.

Aleppo, an ancient city dominated by its landmark citadel, is home to two million people and saw fierce battles earlier in the conflict.

But until this weekend, the militants were never able to totally take over.

AFPTV footage showed militants in military fatigues patrolling Aleppo's streets, some setting fire to a Syrian flag.

Militants have also taken dozens of towns across the north, including Khan Sheikhun and Maaret al-Numan, roughly halfway between Aleppo and Hama, the Observatory said.

AFPTV footage showed insurgents pushing into Hama province in central Syria, as well as army tanks and equipment abandoned on the roadside.

The United States and its allies France, Germany and Britain called Sunday for "de-escalation" in Syria, and for the protection of civilians and infrastructure.

The United States maintains hundreds of troops in northeast Syria as part of an anti-ISIS coalition.

Parallel to the HTS-led offensive, pro-Turkey factions in northern Syria attacked Kurdish fighters in Aleppo province on Sunday, with the Observatory saying they seized the strategic town of Tal Rifaat and nearby villages.

On Monday, in light of the takeover of Tal Rifaat, a US-backed, Kurdish-led force in Syria said it is seeking to evacuate Kurds around Aleppo to safe areas under its control.