WASHINGTON (Reuters) - U.S. Secretary of State Rex Tillerson will meet with senior Turkish officials in Ankara next week, in talks that could be vital to an advancing U.S.-backed campaign to retake the Syrian city of Raqqa from Islamic State, U.S. officials said on Friday.

Turkey has been pressing the United States to drop its military alliance with the Syrian Kurdish YPG militia, which it considers part of the Kurdistan Workers' Party that has been fighting an insurgency for three decades in Turkey.

But U.S. officials have long viewed Kurdish fighters as key to an approaching assault on Raqqa, Islamic State's de facto capital. It would work alongside Arab fighters in the U.S.-backed Syrian Democratic Forces (SDF).

Instead, Turkey wants the United States to draw from Syrian Arab rebel groups backed by Ankara for the final assault on Raqqa, a predominantly Arab city, proposals that so far have failed to convince U.S. officials who are not certain that the Turkish-backed Arab force is large and well-trained enough.

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"We first need to work out details with Turkey," one senior official said, speaking on condition of anonymity.

The decision sets President Donald Trump's wish for quick battlefield victories against the need to maintain the United States' longstanding strategic alliance with Turkey, a NATO ally which provides the United States access to a base critical for the air war in Syria.

The Raqqa campaign appears to be gathering steam as an overlapping U.S.-backed effort in Iraq is drawing closer to driving Islamic State from the city of Mosul.

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French Defense Minister Jean-Yves Le Drian said on Friday that the battle for Raqqa would likely start "in the coming days."

The head of the Syrian Kurdish YPG militia told Reuters last week that the assault would begin at the start of April, and that the YPG would be taking part.

Some U.S. officials think that timeline is too optimistic, noting that a major battle underway for Tabqa dam, about 25 miles (40 km) west of Raqqa, could take weeks to complete.

The Pentagon also has said the United States has not decided on the composition of an American-backed assault force in Raqqa.

The United States has about 1,000 troops in Syria at the moment, and the Raqqa campaign could involve hundreds more.

KURDISH CONUNDRUM

Senator John McCain, the Senate Armed Services Committee's chairman, suggested to reporters earlier this week that it would be hard to convince Turkish President Tayyip Erdogan of the merits of backing Kurdish fighters, even if they are the most effective weapon against Islamic State.

"The conundrum is that if you don't use the Kurds, it will take a lot longer while (Islamic State is) sending streams of terrorists out of Raqqa," McCain said.

"But if you do, you have an enormous challenge as far as relations with Turkey is concerned," he added.

McCain said the relationship with Erdogan was particularly important and cited an April 16 referendum on constitutional changes to extend his powers.

SAFE ZONES

Tillerson's meeting in Ankara also closely follows a meeting of 68 coalition partners this week in Washington, where they agreed to intensify the fight against Islamic State, including in Syria.

At Wednesday's meeting, Tillerson said the United States would create "interim zones of stability" through ceasefires to help refugees return home in the next phase of the fight against Islamic State and al Qaeda in Syria and Iraq.

He did not elaborate, although one U.S. official said there was not yet agreement among all American agencies on where and how such zones would be set up.

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The U.S. military has long been skeptical of creating safe zones inside Syria or Iraq, partly because any safe zone guaranteed by the United States almost certainly would require some American military protection. Securing the ground alone would require thousands of troops, former U.S. officials and experts say.

The top U.S. commander on the ground, Lieutenant General Stephen Townsend, said earlier in March that, "We are building safe zones right now for all Iraqis and all Syrians to be safe from (Islamic State)."

Increased U.S. or allied air power would be required if Trump chooses to enforce "no fly" restrictions and ground forces might also be needed to protect civilians in those areas.

The State Department did not provide more details of Tillerson's trip to Turkey, but said he will visit Brussels on March 31 to participate in a rescheduled meeting of NATO foreign ministers.

The NATO meeting had been set for April 5-6, but it was rescheduled after Tillerson decided instead to attend a summit with China. That caused unease among European allies already worried about Trump's commitment to the Western military alliance.

(Reporting by Lesley Wroughton; Editing by John Walcott and Jonathan Oatis)