[Source: Reuters]

Russia carried out “a series of air strikes” in Syria on Sunday as rebels advanced after seizing Aleppo, according to the UK-based Syrian Observatory for Human Rights (SOHR).

Twelve people were killed in a strike on a hospital in Aleppo, while a strike inside the north-western city of Idlib killed eight civilians and wounded more than 50, SOHR said.

Warplanes also struck rural parts of Idlib and Hama where the group leading the rebel offensive “has recently taken control”, it added.

The Syrian government has meanwhile lost control of Aleppo for the first time since the country’s civil war began, the observers told news agency AFP.

The surprise offensive by opposition forces, which began on Wednesday, marks the most significant fighting in Syria’s civil war in recent years.

It has been led by the Islamist militant group Hayat Tahrir al-Sham (HTS), as well as allied factions backed by Turkey.

More than 300 people, including at least 20 civilians, have been killed so far, according to SOHR.

Russian strikes hit Aleppo for the first time since 2016 on Saturday as rebel fighters swept into Aleppo, Syria’s second-largest city, the observers said.

The Russian air force played a significant role in keeping President Bashar al-Assad in power during the peak of the civil war.

SOHR reported five Russian air strikes in a refugee camp in Idlib early on Sunday, a north-western city in which HTS is the dominant force.

SOHR later reported four strikes on Aleppo University Hospital, with eight civilians among the 12 dead, with other parts of the city also targeted.

Opposition fighters seized nearly all of the city from the government on Sunday, SOHR told AFP – except for the neighbourhoods controlled by Kurdish forces.

Syria’s military withdrew from the city to prepare a counteroffensive.

Meanwhile, the rebels pushed south from Aleppo into several towns in the countryside near Syria’s fourth largest city, Hama, according to SOHR.

Syria’s defence ministry said the force had “reinforced [its] defensive lines” in the region with troops, rocket launchers and heavy military hardware.

It added its troops had recaptured several towns that had been seized and prevented rebels from advancing, and claimed they were fleeing the region.

The situation poses “severe risks to civilians and [has] serious implications for international peace and security”, the United Nations (UN) special envoy for Syria warned.

Geir O Pederson said he had repeatedly warned of the dangers of “mere conflict management rather than conflict resolution” in Syria.

He called the present conflict a “mark of collective failure” to work towards a ceasefire in the country, an aim the UN Security Council endorsed in 2015, demanding “urgent and serious political engagement” to this end.

Speaking on Saturday, President Assad vowed to “defend [Syria’s] stability and territorial integrity in the face of all terrorists and their backers”.