An Islamist-dominated rebel alliance launched its assault on forces of the Iranian- and Russian-backed government on Wednesday, the same day a fragile ceasefire took effect in neighbouring Lebanon between Israel and the Iran-backed Hezbollah after two months of all-out war.

The jihadist Hayat Tahrir al-Sham (HTS) group and allied factions now "control Aleppo city, except the neighbourhoods controlled by the Kurdish forces", Rami Abdel Rahman, head of the Britain-based Syrian Observatory for Human Rights, told AFP.

For the first time since the civil war started more than a decade ago, "Aleppo city is out of control of Syrian regime forces", he said.

The Observatory said Syrian and Russian aircraft staged deadly strikes Sunday in support of the government.

It said at least 12 people were killed in the city, raising an earlier toll of five from attacks targeting the area near Aleppo university.

Russian strikes also killed eight civilians, including two children and a woman, in the rebel bastion of Idlib, the Observatory said.

Idlib resident Umm Mohamed said she lost her daughter-in-law, who left behind five children, including a wounded little girl.

"We were sitting in the room and suddenly we heard the sound of an explosion, the walls fell on us," she told AFP from hospital.

"From the dust, no one could see the others... I was with my son's five children. Thank God their injuries were minor."

In 2016 the Syrian army -- supported by Russian air power -- recaptured rebel-held areas of Aleppo, a city dominated by its landmark citadel.

Damascus also relied on Hezbollah fighters to regain swathes of Syria lost to rebels early in the war, which began in 2011 when the government crushed protests. But Hezbollah has taken heavy losses in its fight with Israel.

- Tanks seized -

Several northern districts inside Aleppo are predominantly inhabited by Syrian Kurds under authority of the People's Protection Units (YPG), the main component of the US-backed Syrian Democratic Forces (SDF).

Before this offensive, HTS, led by Al-Qaeda's former Syria branch, already controlled swathes of the Idlib region, the last major rebel bastion in the northwest.

HTS also held parts of the neighbouring Aleppo, Hama and Latakia provinces.

The latest fighting has killed more than 370 people, mostly combatants but also including at least 48 civilians, according to the Observatory, which has a network of sources inside Syria.

The Observatory said rebel advances met little resistance.

It said Sunday the army strengthened its positions around Syria's fourth largest city Hama, about 230 kilometres (140 miles) south of Aleppo, and sent reinforcements to the north of the surrounding province.

Syria's defence ministry said units in Hama province "reinforced their defensive lines with diverse means of fire, equipment and personnel".

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

- 'Weak' government -

In Idlib on Sunday, bodies lay in a hospital and vehicles were torched in the street, AFP images showed, after what the Observatory said were Russian air strikes.

In Aleppo, an AFP photographer saw charred vehicles. Inside one car, a woman's body lay slumped in the back seat, a handbag beside her.

The Russian air strikes on parts of Aleppo are the first since 2016.

One resident, who asked not to be identified due to safety concerns, told AFP that like most locals, "we are holed up at home".

The rebels "are trying to put on a kind face and reassure the population. They forced the bakeries to work through the night, and distributed free bread at intersections today", the resident added.

Aaron Stein, president of the US-based Foreign Policy Research Institute, said "Russia's presence has thinned out considerably and quick reaction air strikes have limited utility".

He called the rebel advance "a reminder of how weak the regime is".

Aron Lund of the Century International think tank said "Aleppo seems to be lost for the regime... and a government without Aleppo is not really a functional government of Syria".

Syria's "reliance on Russia and Iran", along with its refusal to move forward with a 2015 peace process outlined by the UN Security Council, "created the conditions now unfolding", said US National Security Council spokesman Sean Savett.

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

- Diplomacy -

Iran's Foreign Minister Abbas Araghchi left Tehran for Damascus to deliver what state media said would be a message of support for Assad.

Araghchi again called the rebel offensive a US and Israeli plot, and vowed that "the Syrian army will once again win".

Assad vowed to defeat the "terrorists", however big their attacks.

"Terrorism only understands the language of force, and that is the language which we will break it and eliminate it with, whoever its supporters and sponsors are," he said.

Israeli Prime Minister Benjamin Netanyahu said: "We are constantly monitoring what is happening in Syria."

Russia, whose air support was previously decisive in helping Syria's government win back lost territory, joined Iran in expressing "extreme concern" over their ally's losses.

UN envoy Geir Pedersen said the "latest developments pose severe risks to civilians and have serious implications for regional and international peace and security".

Jordan's King Abdullah II expressed support for Syria's "territorial integrity, sovereignty and stability", and Pope Francis urged a prayer "for Syria, where war has unfortunately reignited, resulting in many victims".