Syrian regime forces on Thursday edged closer to an Islamic State (ISIS) group bastion in the northern province of Aleppo, capturing a nearby town, a monitor said.

Backed by intense Russian air strikes, fighters loyal to President Bashar al-Assad seized Aarran from ISIS militants, according to the Syrian Observatory for Human Rights.

The town lies just 10 kilometres (six miles) south of the IS stronghold of Al-Bab.

"Regime forces are combing the town. There are some light clashes, but it is under government control," said Observatory head Rami Abdel Rahman.

"This is the closest the government has been to Al-Bab since 2012," he told AFP.

Roughly 30 kilometres (25 miles) south of the Turkish border, Al-Bab fell into rebel hands in July 2012, and ISIS militants captured it in late 2013.

Government forces are trying to cut off ISIS territory in Aleppo province from the group's stronghold in Raqa further east, Abdel Rahman said.

Raqa is the de facto Syrian capital of ISIS's self-styled "caliphate", which straddles Syrian and Iraqi territory.

The Aleppo offensive is part of a series of ground operations across the country backed by Assad's ally Russia, which began air strikes in Syria on September 30.

According to Abdel Rahman, at least 63 ISIS fighters have been killed in Russian raids in the Al-Bab region since Saturday.

More than 260,000 people have been killed in Syria's conflict, which broke out in March 2011 with anti-government protests that spiralled into a multi-sided civil war.