A US-backed alliance of Kurdish and Arab fighters has seized 350 square kilometres over the past week in Syria's Raqqa province, "tightening their noose" on ISIL in an advance to isolate its base of operations, a spokesman for a US-led coalition said on Wednesday.

Some 3,000 to 4,000 ISIL fighters are thought to be holed up in Raqqa city where they continue to erect defences against the anticipated assault, Colonel Ryan Dillon told the Reuters news agency in a phone interview from Iraq's capital, Baghdad.

The Syrian Democratic Forces (SDF) has been encircling Raqqa, ISIL's self-proclaimed capital in Syria, since November. Last week, its fighters accomplished a major goal by capturing Tabqa, a previously ISIL-held town some 50km west of Raqqa, and a strategic dam nearby.

"In the last week, the SDF have tightened their noose around ISIL in the northern and eastern and western part of Raqqa," Dillon said.

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Backed by coalition air raids, SDF fighters had drawn as near as four kilometres to Raqqa at their closest point to the city.

"They have taken almost 350 sq km of ground from ISIL, so they continue to just encroach and ... tighten, and tighten and tighten that isolation ring, quite frankly largely uncontested," he said.

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The SDF, which includes the powerful Kurdish YPG armed group, said last week it plans to launch the final assault on Raqqa city in early summer. YPG and SDF officials had previously given April start dates for the assault, but these slipped as the battle for Tabqa dragged on.

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Asked about the timeframe, Dillon said: "We do not try to stick to a timeline.

"There's a principle of war called surprise and we want to achieve that and we want our partner force to achieve that when they do decide it is the right time and place or places ... to start their assault."

While the coalition already has armed Arab fighters in the SDF, the White House last week authorised supplying weapons for the YPG to help in the Raqqa assault.

Dillon said there had been no deliveries and no specifics on the types of weapons and equipment the SDF will receive.

'People are starving'

Meanwhile, people who have returned to Tabqa have found destruction and a shortage of food in the wake of ISIL's departure.

"People are starving and they are fighting each other for bread," Abu Mohammed, a resident of the town, told Al Jazeera.

"They're exhausted; they feel as though they are dying - but thank God, we are saved and we can live again."

Al Jazeera's Charles Stratford, reporting from Gaziantep in neighbouring Turkey, said "there are reports of heavy fighting in the countryside to the north and east of [Tabqa].

"But there are also growing concerns about what the UN describes as hundreds of thousands of people who are expected to flee the fighting as it intensifies in the days and weeks ahead."

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The SDF recently said in a statement that Tabqa would be turned over to a civilian council once fully secured.

It also said the authority that oversees the Tabqa dam would remain "a national Syrian institution that will serve all the regions of Syria without exception".