Russian-backed Syrian government forces battling the Islamic State group in the northern province of Aleppo have neared a key water pumping station and an airbase, a monitor said on Tuesday.

The Al-Khafsah pumping station provides the water supply for Aleppo and residents of the government-held second city have been without mains water for some 50 days since the militants cut it.

"Regime forces are now on the outskirts of the Jarrah military airport and the town of Al-Khafsah and the water pumping station," the Syrian Observatory for Human Rights said.

"There are now no villages left between regime forces and the town of Al-Khafsah or the town of Deir Hafer," the Britain-based monitor said, referring to two IS-held areas.

A government security source confirmed the army had made "significant progress" in the east of Aleppo province, seizing 20 villages and areas on Monday.

The source said the army had neared "the outskirts of Al-Khafsah, and the west bank of the Euphrates river."

"This progress is tightening the noose on Daesh and puts it in difficult circumstances and makes its withdrawal (from the area)... a matter of time," the source added, using an Arabic acronym for IS.

IS militants seized the Jarrah airbase in January 2014 from rebels who had taken it from the government in February 2013.

Shortly afterwards, there were reports the militants were being trained to fly jets they had captured at the base, and the government said it destroyed two of three planes in IS group hands, adding the third was already unusable.

The government offensive in Aleppo province has involved heavy air strikes and shelling and prompted an exodus of civilians.

The UN says at least 26,000 have fled since February 25, with many heading towards the town of Manbij and surrounding villages, which are held by a US-backed Kurdish-Arab alliance known as the Syrian Democratic Forces.