Syrian activists say the Islamic State group has launched a surprise attack in the northern province of Aleppo, seizing a string of villages from rebels.

The Syrian Observatory for Human Rights says the advance by the extremists is the most significant in months.

It comes amid a wave of Russian airstrikes and a ground offensive by the Syrian army in the country's central region.

The Observatory said IS seized the villages of Tal Qrah, Kfar Qares and at least four other small villages in the northern Aleppo countryside. IS-affiliated accounts on Twitter also announced the militants had seized those villages. (AP)