Fierce fighting took place on Wednesday between jihadist militants and US-backed Syrian rebels just 20 kilometres (12 miles) from the Islamic State's self-proclaimed capital of Raqa, an AFP reporter saw.
On Saturday, the rebels -- a coalition of Arab and Kurdish fighters known as the Syrian Democratic Forces (SDF) -- announced a new phase in a bid to capture Raqa, and mounted an attack from north and northeast.
An AFP reporter at Bir Fawaz, 20 km north of Raqa, heard machine-gun fire all day Wednesday as SDF attacked IS positions in the neighbouring village of Maayzila.
The village also came under repeated attack by at least three anti-IS coalition warplanes, and heavy smoke could be seen billowing from there.
"There has been heavy fighting since the morning. Many IS fighters are holed up inside Maayzila," the local SDF commander, Ararat Kojer, told AFP.
Twelve villages have fallen to the rebels in the latest phase of the offensive, which was launched on November 6.
"Progress is slow because IS are putting up a ferocious resistance and placed mines around the approaches to many villages," Rami Abdel Rahman of the Syrian Observatory for Human Rights, which is monitoring the conflict, told AFP.
The SDF, which has been lobbying for weapons to help them carry out the offensive, has recently received armoured SUVs from the United States.
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A US-led coalition has been carrying out air strikes on IS in Syria and neighbouring Iraq since 2014.
The SDF's offensive is being supported by 500 US troops, including demining teams, military trainers and members of the special forces.
Earlier Wednesday, a spokesman for the coalition predicted that the IS bastion would soon be almost cut off.
Although it will not be completely encircled, "it will be very difficult to get into or out of the city," Colonel John Dorrian said in a video conference from Baghdad.
"What we would expect is that within the next few weeks the city will be nearly completely isolated," Dorrian said.
US-backed Iraqi forces have wrested part of the Iraqi city of Mosul from IS control, although the city's western districts have yet to be retaken.
Raqa is the coalition's next big objective. But the issue of who exactly will carry out the assault has not yet been worked out.
Turkey has expressed interest in taking part in the operation, with Turkish Foreign Minister Mevlut Cavusoglu indicating that his country is ready to deploy special forces to take part in the battle.
Turkey opposes giving added weight to SDF, viewing it as little more than a front for the Kurdish YPG militant group, which Ankara considers a terror organisation.