Thousands of civilians fled a key town in northern Syria held by the Islamic State group early on Tuesday as US-backed Kurdish and Arab fighters closed in, a monitor said.

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 kilometres (three miles) of Manbij from the north," Observatory head Rami Abdel Rahman told AFP.
Late on Monday, SDF fighters had pushed to within two kilometres (just over a mile) of the town to the south and around seven kilometres (five miles) to the east.
"Daesh has begun allowing civilians to flee towards the west, whereas before they had banned anyone from leaving," Abdel Rahman said, using an Arabic acronym for IS.
He said IS fighters were holding their positions but thousands of residents had left.
Manbij lies at the heart of the last stretch of territory along the Turkish border still under IS control.
The SDF's offensive is aimed at seizing it and denying the jihadists any opportunity to smuggle recruits or funds across the border.
Washington, which has more than 200 special forces troops deployed with the SDF, has said some 3,000 Arab fighters are taking part in the assault, backed by around 500 Kurds.
The US-led coalition battling IS in Iraq and Syria has carried out heavy air strikes in support of the offensive.
The jihadists are facing multiple assaults in Syria.
Further down the Euphrates Valley, both the SDF and Russian-backed regime forces are advancing from opposition directions towards Syria's largest dam and the adjacent IS-held town of Tabqa.
If the rival offensives meet up, it would cut the Manbij border pocket off from IS's de facto Syria capital Raqa downstream.