Thousands of civilians fled a key town in northern Syria held by ISIS early on Tuesday as US-backed Kurdish and Arab fighters closed in, AFP reports.

The Syrian Democratic Forces are advancing on Manbij from three directions after crossing the Euphrates River from their northeastern stronghold last week in a major offensive against the jihadists, the Syrian Observatory for Human Rights said.

"Overnight, they advanced to within five kilometers of Manbij from the north," Observatory head Rami Abdel Rahman told AFP.

Late on Monday, SDF fighters had pushed to within two kilometers of the town to the south and around seven kilometers to the east.