BEIRUT (Reuters) - The U.S.-backed Syrian Democratic Forces (SDF) have seized 350 square km (135 square miles) in the past week, tightening “their noose” on Islamic State in an advance to isolate its base of operations at Raqqa, a spokesman for the U.S.-led coalition said on Wednesday.

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Some 3,000 to 4,000 Islamic State fighters are thought to be holed up in Raqqa city where they continue to erect defenses against the anticipated assault, drawing coalition air strikes to stop them, Colonel Ryan Dillon, a spokesman for the U.S.-led coalition, told Reuters in a phone interview from Baghdad.

The SDF, an alliance of Kurdish and Arab fighters, has been encircling Raqqa since November in a multi-phased offensive backed by the U.S.-led coalition that is also fighting Islamic State in Iraq. Last week, the SDF accomplished a major goal by capturing Tabqa some 50 km (30 miles) west of Raqqa.

“In the last week, the SDF have tightened their noose around ISIS in the northern and eastern and western part of Raqqa,” Dillon said. SDF fighters had drawn as near as 4 km (2.5 miles) to Raqqa at their closest point to the city.

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

The SDF, which includes the powerful Kurdish YPG militia, said last week it expects 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 went on.

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 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.”

Defenses being erected by Islamic State include earthen berms, some of them designed to hold large pools of water. In some cases, these had collapsed, causing flooding in Raqqa city.

While the coalition already has armed Arab fighters in the SDF, the White House last week authorized 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.

Turkey sees the YPG as an extension of the Kurdistan Workers’ Party (PKK) that has fought a three-decade insurgency for Kurdish autonomy and rights inside Turkey.

(This story was refiled to add dropped word “on” in headline)