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).

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.

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.