(Adds fighters' deaths)
BEIRUT, Aug 13 (Reuters) - Islamic State insurgents have seized several towns and villages from rival Islamist groups in the Syrian province of Aleppo, opening the way for further westward advances, an organisation monitoring the war in Syria said on Wednesday.
Already in control of large areas of northern and eastern Syria, Islamic State's latest gains include the towns of Turkmen Bareh and Akhtarin, 50 km (30 miles) northeast of Aleppo, the Syrian Observatory for Human Rights, which is based in Britain, reported.
It said at least 40 fighters were killed during the fighting.
Islamic State's advance in Syria has accelerated since the group seized control of the Iraqi city of Mosul in June, declaring a caliphate in areas under its control in a bid to redraw the borders of the Middle East.
Diplomats and rebels said the Syrian government, which is fighting rebels across the country, launched attacks on towns run by the group only after its militants advanced into neighbouring Iraq and seized a third of its territory.
Islamic State is tightening its grip over areas of Syria under its control, including the city of Raqqa on the Euphrates river. Raqqa has become Islamic State's Syrian power base.
The Observatory, which uses a network of monitors, reported battles between Islamic State fighters and Syrian government forces near a Raqqa military airport, the government's last remaining position in the area.
The Observatory also reported two more crucifixions by Islamic State in Deir al-Zor overnight, bringing to 27 the number of people executed by the group in recent days as it tightens it grip over the eastern province.
The advance in northeast Aleppo province is part of a wider Islamic State offensive to seize a belt of territory near the border with Turkey.
A Twitter feed reporting events in Akhtarin said two people had been killed while "resisting" Islamic State's advance, including a 16-year-old boy. There was no way to verify the report.
The Observatory said it expected Islamic State's next targets to be the towns of Azaz and Marea to the west.
It identified the groups defeated by Islamic State as fighters from Islamist battalions that have been resisting its expansion. The Nusra Front, al Qaeda's official affiliate in Syria, and other Islamist groups had withdrawn from the area in June. (Addditional reporting by Omar Fahmy in Cairo; Writing by Tom Perry; Editing by Janet Lawrence)