A US-backed alliance of Kurdish and Arab fighters has reportedly advanced into a strategically important Syrian town held by so-called Islamic State.
The Syrian Observatory for Human Rights said there were heavy clashes between the Syrian Democratic Forces and IS militants on the outskirts of Manbij.
The SDF began an offensive to take the town on 31 May, quickly encircling it.
Manbij sits on an intersection of roads linking the IS stronghold of Raqqa to the Turkish border and Aleppo province.
The SDF, which is dominated by members of the Kurdish Popular Protection Units (YPG), has already driven IS out of much of north-eastern Syria and controls more than 500km (310 miles) of the border.
The Syrian Observatory, a UK-based monitoring group that relies on a network of sources on the ground, reported on Thursday that clashes were taking place for the first time inside Manbij.
SDF fighters were pushing into a western part of the town, between the Kitab and Sharia roundabouts, supported by US-led coalition air strikes, it said.
IS militants had rigged houses with booby-traps, slowing the advance, it added.
A spokesman for the US-led coalition, Col Chris Garver, told reporters on Wednesday that it anticipated a "good fight" once SDF fighters entered Manbij.
"We've anticipated that they [IS] would hold Manbij till the last, that it would be one of those places that they defended till the end and we have not seen anything that's going to change that assessment right now," he said.
The SDF said it had thwarted an IS counter-attack on Monday that included the use of vehicles laden with explosives and reportedly inflicted heavy casualties.
At least two SDF fighters were killed in Thursday's fighting, bringing to 63 the total number killed since the Manbij offensive began, according to the Syrian Observatory. At least 458 IS militants are said to have been killed in that time.
The SDF completely encircled Manbij almost two weeks ago, but held back from launching a final assault to retake it out of concern for civilians inside the town.
The Syrian Observatory said more than 3,000 civilians had fled Manbij during the fighting, but the UN warned on 6 June that as many as 236,000 might be displaced.