Syria has said it is opening a corridor for civilians to leave the rebel-held northwestern region of Idlib, where an intense bombardment by government forces has killed hundreds of people since April.

The announcement came a day after government forces captured the key Idlib province town of Khan Sheikhun from jihadists and allied rebels.

Syria has opened such corridors out of other rebel-held areas in the past as a prelude to retaking them either by force or through negotiated surrenders.

The Idlib region, which sits on the Turkish border, is now the last major stronghold of opposition to the Russia-backed government of President Bashar al-Assad.

The Syrian Observatory for Human Rights, a British-based group, said yesterday's advance saw government forces cutting off a pocket of territory stretching from the south of Idlib province into neighbouring Hama.

The humanitarian corridor will be used to remove civilians who want to leave areas controlled by terrorists in northern Hama and the southern countryside of Idlib.

The Idlib region has been ruled since January by the Hayat Tahrir al-Sham (HTS) alliance, which is led by jihadists from Syria's former al-Qaeda affiliate.

The Syrian government refers to all rebel groups in Idlib as "terrorists".

The region of some three million people was supposed to be protected by a proposed buffer zone agreed by Russia and Turkey last September.

But the jihadists of HTS failed to pull back from the zone as agreed and in April government and Russian forces resumed intense bombardment of the region.

Around 890 civilians have been killed, according to the Syrian Observatory for Human Rights.

More than 400,000 more have fled their homes, the United Nations said.

The entry of government forces into Khan Sheikhun raises the stakes between Syria and Turkey, which has troops deployed in the nearby town of Morek, which is now cut off.

The war in Syria has killed more than 370,000 people since it started with the brutal suppression of anti-government protests in 2011.