Thousands of civilians have fled a stronghold of the Islamic State group in northern Syria almost surrounded by US-backed fighters seeking to cut a key supply lifeline for the jihadists.

The Arab-Kurdish offensive on the town of Manbij is one of two major assaults on the route IS uses to send in more fighters, weapons and money from the Turkish border to its main Syrian bastion of Raqa.
"We have surrounded Manbij from three sides and operations are progressing well," Sherfan Darwish, who is leading the offensive of the Syrian Democratic Forces launched just over a week ago, said on Tuesday.
"Every day, we are liberating villages and the only route open to IS now is towards Aleppo city" to the west, Darwish told AFP by phone.
The SDF is now within five kilometres (three miles) of Manbij from the north, two kilometres from the south and about seven kilometres from the east, said the Syrian Observatory for Human Rights.
"Daesh has begun allowing civilians to flee towards the west," some in cars but many carrying their belongings on foot, the Observatory's Rami Abdel Rahman said, using an Arabic acronym for IS.
About 20,000 people are still living in Manbij, which had a pre-war population of about 120,000 -- mostly Arabs, but about a quarter Syrian Kurds.
IS overran the town in early 2014, just months before declaring a cross-border "caliphate" in Iraq and Syria.

Vital supply route

The "Manbij pocket" was the only remaining section of territory used by IS to smuggle recruits or funds from Turkey across the border.
The US-led coalition battling IS in Iraq and Syria has carried out heavy air strikes to support the fight for Manbij.
Warplanes operating from the USS Harry S. Truman aircraft carrier in the eastern Mediterranean have mounted at least 35 strikes on IS since relocating last week, its officers told AFP on Tuesday.
And the US military said the coalition had freed a total of 344 square kilometres (133 square miles) from IS since the operation to retake Manbij was launched on May 31.
The jihadists' supply route leads from Jarabulus on the border south through Manbij and winds southeast along the Euphrates through the town of Tabqa and on to Raqa city.
Washington, which has more than 200 special forces troops deployed to back the SDF, has said some 3,000 Arab fighters are taking part in the assault, supported by about 500 Kurds.
The Observatory has said that of the 4,000 fighters it estimates are taking part, most are actually Kurdish.
The SDF advance on Manbij has cut the route to Jarabulus to the north and to Raqa province to the south, Darwish told AFP.
Tabqa, another IS-held transit town which also lies near Syria's largest dam, is also under attack on two fronts.
SDF fighters began advancing on Tabqa last week, but are still 60 kilometres away from the riverside town.

 'No choice but victory'

Russian-backed regime fighters have steadily advanced from the town's southwest, and are just over 30 kilometres away, a Syrian military source told AFP.
The Observatory said IS had sent a convoy of weapons and about 100 fighters to reinforce Tabqa in preparation for a major battle.
As fighting for Tabqa and Manbij intensifies, it appears the battle for IS's de facto capital of Raqa city -- which would be a much more symbolic victory -- has taken a backseat.
The US-backed SDF's offensive north of Raqa last month began amid much fanfare, but progress appears to have slowed.
Syria's conflict has evolved into a complex war involving foreign powers since starting in 2011 with the brutal repression of anti-government protests.
Peace talks to end the five-year war -- which has killed more than 280,000 people and displaced millions -- have stalled and a related ceasefire is in tatters.
But President Bashar al-Assad hardened his position on the UN-backed talks in his first address to the newly elected parliament on Tuesday.
"As we liberated Palmyra, so shall we liberate every inch of Syria... we have no choice but victory," he said.
The United States said Assad's vow to recapture the whole of the country was discouraging, urging Russia and Iran to pressure their ally into respecting a ceasefire.
US State Department spokesman Mark Toner said the belligerent speech was unsurprising and dubbed it "vintage Assad".