BEIRUT • Fighters from the Islamic State in Iraq and Syria (ISIS) group advanced yesterday on Syria's second city Aleppo despite 10 days of Russian air strikes that Moscow says are aimed at routing the militants.

The militants' gains came as regime forces, backed by Russian bombing and fighters from the Lebanese Shi'ite militia Hizbollah, intensified an offensive against rebels in the north-west, where ISIS is absent.

Western governments say the vast majority of Russian strikes have targeted rebel groups other than ISIS in a bid to defend President Bashar al-Assad's rule.

ISIS militants reached their closest position yet to Aleppo in northern Syria at dawn yesterday after hours of ferocious fighting with rival opponents of Mr Assad, a monitoring group reported.

"Dozens of combatants were killed on both sides," said Mr Rami Abdel Rahman of the Syrian Observatory for Human Rights.

The extremists are now just over 10km from the northern edges of Aleppo city, edging closer to the front line, where pro-regime forces are positioned, including the Sheikh Najjar industrial zone.

"This is (ISIS') biggest advance towards" the country's pre-war commercial capital, said Mr Abdel Rahman, whose Britain-based group relies on a network of sources on the ground across Syria.

Control of Aleppo city is divided between rebel groups in the east, and government forces, bolstered by pro-regime militias, in the west.

ISIS has not had a presence in the city, but the radical group yesterday boasted it had "reached the gates of Aleppo".

Mr Thomas Pierret, an expert on Islam in Syria, said the US-led coalition bombing ISIS in Syria was "not very active" in Aleppo, and that Russia's strikes there had struck mostly rebels, allowing ISIS to push forward.

AGENCE FRANCE-PRESSE