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

They hit rural parts of Idlib and Hama where the group leading the rebel offensive “has recently taken control”, SOHR said in an update.

It follows the first Russian strikes on Aleppo since 2016 on Saturday, as rebels seized most of Syria’s second-largest city.

The offensive, which began earlier this week, marks the most significant fighting in Syria’s civil war in recent years.

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

Opposition fighters pushed into several towns in the countryside near Syria’s fourth largest city, Hama – south of Aleppo – on Saturday, they said.

The military has now “drawn a defensive line” around a number of cities and towns in the region, SOHR wrote on X.

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

“[The country] is capable, with the help of its allies and friends, of defeating and eliminating them, no matter how intense their terrorist attacks are,” his office quoted him as saying.

Iran’s top diplomat Abbas Aragchi is set to visit Damascus on Sunday to discuss the offensive.

The civil war, which has left around half a million people dead, began in 2011 after the Assad government responded to pro-democracy protests with a brutal crackdown.

The conflict has been largely dormant since a ceasefire agreed in 2020, but opposition forces have maintained control of the north-western city of Idlib and much of the surrounding province.

Idlib sits just 55km (34 miles) from Aleppo, which itself was a rebel stronghold until it fell to government forces in 2016.

The latest offensive has been led by an Islamist militant group known as Hayat Tahrir al-Sham (HTS) and allied factions backed by Turkey.

HTS was regarded as one of the most effective and deadly of the groups fighting the Assad government and was already the dominant force in Idlib.

The rebels have taken control of Aleppo’s airport and dozens of nearby towns, according to the SOHR.

They also announced an overnight curfew which came into force at 17:00 local time (14:00 GMT).

The SOHR also said the rebel fighters had advanced south toward Hama and that the Syrian army had withdrawn.

But a military source quoted in Syrian state media on Saturday night disputed this claim.

The Syrian army said rebels had launched “a broad attack from multiple axes on the Aleppo and Idlib fronts” and that battles had taken place “over a strip exceeding 100km (60 miles)”.

Dozens of its soldiers have been killed, it said.

The Russian air force, which played a significant role in keeping Assad in power during the peak of the civil war, carried out air strikes in Aleppo on Saturday.

The strikes marked the first Russia has staged in the city since helping Syrian government forces recapture it in 2016.

Later on Saturday, nine other Russian strikes were carried out on Idlib, SOHR said.

A US spokesperson said Syria’s “reliance on Russia and Iran”, along with its refusal to move forward with a 2015 UN Security Council peace plan, had “created the conditions now unfolding” in the country.

Pictures showed the roads leading out of Aleppo jammed with cars on Saturday as people tried to leave, and smoke rising out of the city’s skyline.