Rebel forces in Syria have taken control of several neighbourhoods in the country's second-largest city, Aleppo, according to the UK-based monitoring group Syrian Observatory for Human Rights (SOHR).

Syrian authorities closed Aleppo airport as well as all roads leading into the city on Saturday, three military sources told Reuters, as rebels opposed to President Bashar al-Assad said they had reached the heart of Aleppo.

The opposition fighters, led by the Islamist militant group Hayat Tahrir al-Sham, carried out a surprise sweep through government-held towns this week and reached Aleppo nearly a decade after having been forced out by Assad and his allies.

As of Friday evening, SOHR said rebels were in control of more than half of the city.

The move is the biggest offensive against the Syrian government in years and the first time rebels fighting the forces of President Bashar al-Assad have reached Aleppo since being forced out by the army in 2016.

Video posted on a channel affiliated with the Islamist militant group Hayat Tahrir al-Sham (HTS) appears to show rebel fighters in vehicles inside the city.

Government forces meanwhile say they have regained positions in a number of towns in Aleppo and Idlib provinces, following an offensive launched by HTS and allied factions on Wednesday.

More than half a million people have been killed in the civil war that erupted after the government cracked down on pro-democracy protests in 2011.

An array of armed groups opposed to the Assad government - including jihadists - took advantage of the turmoil to seize swathes of territory.

The Syrian government - with help from Russia and other allies - later retook most the areas it had lost.

Idlib, the last remaining opposition stronghold, is mostly controlled by HTS, but Turkish-backed rebel factions and Turkish forces are also based there.