Syrian rebels have swept through parts of the country at lightning pace, taking control of the the second-largest city, Aleppo. But who are they and what are their aims?

The Syrian regime has rushed in reinforcements to hold off a rebel advance into the Hama countryside, after the insurgents seized the country’s second city Aleppo in the biggest threat to autocratic ruler Bashar al-Assad in years.

The lightning assault on Aleppo by the Islamist militant group Hayat Tahrir al-Sham abruptly redrew the front lines of Syria’s civil war.

The Syrian military has rushed in reinforcements to push back insurgents from advancing farther into the northern Hama countryside after they seized Aleppo in a surprise offensive

The surprise attack, led by the Islamist group Hayat Tahrir al-Sham, was the boldest rebel assault for years in the civil war that has been frozen.

Iran, a key ally of President Bashar Assad, has pledged to help the government counter the surprise offensive.

Syrian rebels launched a surprise attack on Saturday and quickly advanced through several provinces in the north and northwest of Syria, including the city of Aleppo, the country’s second largest city.

At least 372 people have been killed since rebels launched a surprise offensive on Wednesday, according to observers.