Syrian government forces have advanced towards so-called Islamic States last major stronghold in Aleppo province, cutting off the main supply route into al-Bab, a monitoring group reports.

The Syrian Observatory for Human Rights said troops had captured Tal Uwayshiya, a hill 9km (6 miles) to the south-east.

It overlooks a key road linking al-Bab to the IS-controlled city of Raqqa, in a neighbouring province.

Al-Bab is now almost surrounded by Turkish-backed Syrian rebels.

The rebels are laying siege from the north, east and west.

It is not yet clear whether the Syrian government is also aiming to capture the town, which is considered a prize by all sides in the five-year civil war.

But a military source told the pro-government al-Masdar website that troops were now within striking distance of the village of Tadif, on the outskirts of al-Bab.

The government has denounced Turkeys support of the rebel offensive, which was launched in August with the aim of clearing IS militants from a border area north of al-Bab and limiting gains to the east by Kurdish forces.

Despite that, President Bashar al-Assads ally Russia has carried out joint air strikes with Turkey on IS positions around al-Bab since January.

The Syrian Observatory also reported on Monday that Turkish troops and rebel fighters were engaged in deadly clashes with IS militants around the village of Bazaa, east of al-Bab, which was recaptured by the jihadists over the weekend.

Government forces were also battling IS in the central province of Homs, after capturing the Hayyan oilfield west of Palmyra, the UK-based monitoring group said.

It comes as a US-backed alliance of Kurdish and Arab fighters begins a new phase of their offensive to isolate Raqqa.

The Syrian Democratic Forces announced on Saturday that it aimed to take areas east of the city, including the main road linking it to Deir al-Zour province, almost all of which is controlled by IS.

The operation is being supported by air strikes by the US-led multinational coalition against IS and several hundred special forces personnel on the ground.