Supported by air strikes by the US-led coalition battling IS in Syria and Iraq, the Syrian Democratic Forces (SDF) alliance of Kurdish and Arab militias launched an assault last week on Manbij.
UK troops blast Bollywood songs as new weapon against Islamic State
"The Syrian Democratic Forces are now within about five kilometres of the strategic city of Manbij," said the Syrian Observatory for Human Rights, a Britain-based monitoring group.
Manbij is located along a route connecting Raqa -- the IS group's de facto capital in Syria -- to the Turkish border, a vital conduit for supplies and foreign fighters.
US Central Command spokesman Colonel Patrick Ryder said on Saturday that SDF fighters had seized more than 100 square kilometres (40 square miles) of territory during the advance.
More than 55 air strikes have been carried out since the offensive began last Monday, he said, adding that the goal was to hamper IS's ability "to move fighters, weapons, finances (and) supplies into and out of Syria and Iraq."
US embassy says has information on planned terrorist attacks in South Africa
Some 3,000 Arab fighters were taking part in the offensive, backed by around 500 Kurdish militia members, he said, adding that US special forces were working "at the command and control level" in the operation.
Russian-backed Syrian troops are also advancing against IS in Raqa and on Saturday pushed into the province from the southwest, moving to within 40 kilometres (25 miles) of the Euphrates Valley town of Tabqa, site of the country's biggest dam.
COMMENTS
Comments are moderated and generally will be posted if they are on-topic and not abusive.
For more information, please see our Comments FAQ