WASHINGTON (NYTimes) - The fight to retake Raqqa, the Syrian city that serves as the capital of the Islamic State in Iraq and Syria (ISIS), must begin soon - within weeks - to disrupt planning believed to be underway there to stage terrorist attacks on the West, senior Defence Department and military officials said.

Lt. Gen. Stephen Townsend, the top US military commander in Iraq, declined to name a specific threat against Western targets emanating from Raqqa, but described a "sense of urgency."

He said on Wednesday (Oct 27) that it was imperative that operations to isolate the city begin soon to prevent attacks on the West that could be launched from or planned inside the militants' capital.

In announcing that the fight to retake Raqqa is imminent, US officials are sweeping aside objections from Turkey and moving forward with plans to rely on a ground fighting force that includes Kurdish militia fighters in Syria.

The Turkish government, which has become a complicated ally in the fight against the Islamic State, fears aspirations for autonomy may spread among its own Kurdish population.

Lt. Gen Townsend stressed that Kurdish militia fighters will be a part of the ground force used to isolate Raqqa.

"We're going to go with who can go, who's willing to go soon," he told reporters at the Pentagon during a video news briefing from Baghdad. "And then, once we get the initial isolation in position, we'll look at how we prosecute the operation further."

The politics of the matter have bedeviled US military planners trying to thread a needle through decades-old enmity between the Turks and the Kurds. Turkey regards the Syrian Kurdish fighters, known collectively as the YPG, as an extension of the Kurdistan Workers' Party, the Kurdish rebel group that has sought autonomy from Turkey since the 1980s. Ankara has demanded that the YPG not take part in the fight to retake Raqqa.

But US military officials say the YPG are the best fighters they have.

"The facts are these," Lt. Gen Townsend said. "The only force that is capable on any near-term timeline is the Syrian Democratic Forces, of which the YPG are a significant portion."

While the Kurdish militia will make up the bulk of the operation, he said many of the more than 300 US Special Operations forces now in Syria would help recruit, train and equip local forces in and around Raqqa who are predominantly Syrian Arabs.

The impending operation is further complicated, some independent experts say, because neither the Turks nor the Syrian Kurds view the recapture of Raqqa as one of their top priorities - unlike Washington.

"The Syrian Kurdish YPG do not truly desire to shed blood to capture a majority-Sunni Arab city far from their vision of their autonomous borders, while Turkey cares about the operation only insofar as the Syrian Kurdish YPG is not allowed to participate in it," said Dr Christopher Kozak, a Syria researcher at the Institute for the Study of War.

Defence Secretary Ashton B. Carter said in Brussels on Wednesday that the offensive to oust the ISIS from Raqqa would begin within weeks.

"This is, as always, a matter when you're positioning forces and so forth, we have a plan to do that and a schedule to do that," he told reporters. "We're going to execute to that plan."

Mr Carter met with the Turkish Defence Minister Fikri Isik and French Defence Minister Jean-Yves Le Drian in Brussels on Wednesday, where he emphasised the "need for the coalition to maintain pressure" on ISIS on multiple fronts," according to a Pentagon news release.

"All sides agreed to maintain frequent communication on the full range of security interests, and to continue their close coordination and continued transparency in the coalition effort" to deal ISIS a lasting defeat," the statement said.

A US military official said the Raqqa operation would take place in roughly three phases.

Phase one, he said, is what the US-led coalition fighting the ISIS has been doing for months: preparatory airstrikes in and around Raqqa to knock out command-and-control and fighting positions.

Phase two, to begin in the coming weeks, will be to isolate Raqqa with the available forces - mostly Syrian Kurds, with Syrian Arabs, too. Phase three will be the fight for Raqqa itself, which US officials say they hope will be conducted mostly by Syrian Arabs, given that the city is majority Sunni Arab.

Lt. Gen Townsend compared the ISIS plotting in Raqqa to planning by the group in Manbij, which was retaken from the Islamic State in August.

Manbij was the last stop on the route out of Syria for ISIS militants headed to Europe. But the actual plotting regularly began in Raqqa. Militants moved from there to Manbij before slipping over the border into Turkey and then onward to Europe.

Plotting underway in Raqqa now, Lt. Gen Townsend said "is not unlike what emanated from Manbij" before that city was retaken.

"Coming out of Manbij, we found links to individuals and plot streams to France, the United States, other European countries," he said.

"We know that this is going on in Raqqa as well," Lt. Gen Townsend said. "And so I think that's why it's necessary to get down there to Raqqa."

The Raqqa fight will take place even as the fight for Mosul, next door in Iraq, is ongoing, US military planners say.

Gen Joseph L. Votel, the commander of US forces in the Middle East, said last week that it was "extraordinarily important" to keep simultaneous pressure on Mosul and Raqqa, if not with ground forces, as with Mosul, at least with a steady pounding of airstrikes.

He acknowledged the challenges of dealing with two pivotal allies in the fight against the Islamic State in Syria who essentially loathe each other - the Turks and the Syrian Kurds.

One of his main goals now, he said, is to maintain momentum and "to keep everyone moving in the right direction."