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AMMAN (Reuters) - U.S.-backed Syrian forces were fighting through Islamic State defenses on the edge of Manbij city, a militant stronghold near the Turkish border, a senior military official with the American-led coalition fighting the group said on Thursday.

The forces encountered improvised explosive devices and rocket positions, the official said, as they tried to cut off an area that provides the militants with their main access route to the outside world.

“So there is still a civilian population, there are Daesh in defensive areas and the Syrian Democratic Forces are moving closer to them,” said British Army Major General Doug Chalmers, deputy commander for strategy and sustainment with the U.S.-led coalition, using an Arab acronym for Islamic State. He was speaking with reporters in Washington via video link.

The comments come after the British-based Observatory for Human Rights said on Thursday that U.S. backed forces fought Islamic State militants inside the city of Manbij for the first time since they laid siege to the militant stronghold near the Turkish border.

The monitor said heavy clashes were taking place in western districts of Manbij after the fighters swept into the city near the Kutab roundabout, almost 2 km (1.2 miles) from the city center.

“The reporting I’ve had puts them on the edge and the outskirts for some areas which I describe as the outer (part) of the city rather than city proper,” Chalmers said.

The Syria Democratic Forces (SDF), including a Kurdish militia and Arab allies that joined it last year, launched the campaign late last month with the backing of U.S. special forces to drive Islamic State from its last stretch of the Syrian-Turkish frontier.

If successful it could pave the way for an assault on their Syrian capital Raqqa.

Manbij is in a region some 40 km (25 miles) from the Turkish border and since the start of the offensive on May 31, the SDF has taken dozens of villages and farms around it but had held back from entering the city with many thousands of people still trapped there.