U.S.-backed fighters on Thursday closed all major roads leading to the northern Syrian town of Manbij, a stronghold of the Islamic State group, and surrounded it from three sides, officials and Syrian opposition activists said.

The town is one of the largest areas held by ISIS in the northern Aleppo province. Many of its residents fled in advance of the upcoming battle.

Manbij is a waypoint on an ISIS supply line between the Turkish border and the extremist group's de facto capital, Raqqa. If the U.S.-backed Syria Democratic Forces capture Manbij, it will be the extremists' biggest defeat in Syria since government forces captured the central historic town of Palmyra in March.

The U.S. Central Command said the operation to free Manbij is part of the "moderate Syrian opposition" efforts to clear areas along the border with Turkey from ISIS. Members of the American and French military have been advising forces fighting ISIS in northern Syria.

A statement by the Military Council of the City of Manbij, which is part of the SDF, said that all roads from the east, north and south have been cut. The group said they are now close enough to target IS inside the town, but they are holding off storming Manbij to avoid civilian casualties.

The Britain-based Syrian Observatory for Human Rights said SDF fighters are about 800 yards from the last main road linking Manbij with the city of Aleppo, saying that the road is now closed by fire fights.

The Observatory said that since the SDF offensive began on May 31, 132 ISIS fighters, 21 SDF fighters and 37 civilians have been killed.

Mustafa Bali, a Syrian journalist who visited the front lines in Manbij on Thursday, told The Associated Press that the extremists don't appear to be preparing to withdraw from Manbij as the had from other areas. He added that on Wednesday black clouds covered the city as ISIS set tires alight to apparently obscure visibility inside Manbij and prevent airstrikes by the U.S.-led coalition planes flying overhead.

"Daesh is preparing for a battle inside the city," Bali said, using an Arabic acronym to refer to ISIS.

SDF official Nasser Haj Mansour said on Wednesday that some 15,000 civilians had fled Manbij.

The U.S. Central Command said that since the start of their offensive to liberate Manbij, SDF's Syrian Arab Coalition had freed 344 square kilometers from ISIS control. It said that the Combined Joint Task Force Operation Inherent Resolve has conducted more than 105 strikes in support of the battle to liberate Manbij.

"The Syrian Arab Coalition is leading the operation and will be responsible for securing Manbij once it is freed," the statement said. It was an apparent attempt to calm Arab residents of Manbij, who fear that the Kurdish fighters, who are predominant in the SDF, will also enter the town.

The statement said coalition advisers are assisting the fighters in the battles "with command and control from nodes located behind the forward line of friendly forces."

It said the U.S.-led coalition airpower had destroyed 108 ISIS fighting position, 31 vehicles, 17 heavy weapons, two weapons caches and one vehicle borne improvised explosive device.

In France, an official confirmed that French special forces are offering training and giving advice to SDF fighters. The official with the French Defense Ministry said its forces are with SDF fighters who are fighting against ISIS.

The official spoke on condition of anonymity because he was not authorized to speak on the record. He did not provide other details.

In a round-table interview last week, French Defense Minister Jean-Yves Le Drian said French forces were participating. "We are helping with arms, we are helping with aerial support, we are helping with advice," he said.

The U.S. also has around 300 Special Forces embedded with the SDF in northern Syria.

Also Thursday, the U.N. Syria envoy Staffan de Mistura said that the Syrian government had granted approval for humanitarian aid to be delivered to 19 U.N.-designated "besieged areas" in Syria by the end of the month. He cautioned that having these approvals granted would not automatically translate into actual aid deliveries. In the past, aid convoys have been stopped last minute or had some cargo taken off.