COPENHAGEN (Reuters) - U.S. President Donald Trump’s defense secretary emerged upbeat on Tuesday from talks with a Turkish official about the fight against Islamic State but did not disclose any progress toward settling a row over U.S. backing for Kurdish fighters in Syria.

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Turkey, a NATO ally, is adamant that Washington should switch support for the planned assault on the Syrian city of Raqqa from the Kurdish YPG militia to Syrian rebels Turkey has trained and led against Islamic State for the past year.

For the U.S. administration, which is skeptical that the Turkish-backed force is large enough or sufficiently trained, the decision sets Trump’s wish for quick battlefield victories against the need to maintain its strategic alliance with Turkey.

The United States has not yet disclosed a decision on whether it will arm YPG fighters to take Raqqa, Islamic State’s de facto capital, and Jim Mattis declined to answer that question after talks with U.S. allies in Denmark.

“Our intent is to work with the Turks, alongside one another, to take Raqqa down, and we’re going to sort it out and we’ll figure out how we’re going to do it,” Mattis told a news conference, after defense talks with members of the U.S.-led coalition battling Islamic State.

A U.S. official, speaking on condition of anonymity, clarified that Mattis was not signaling a new agreement on a Turkish role in the offensive or any decision on whether to arm the Kurdish fighters.

Pressed about the matter, Mattis said he did not want to disclose U.S. battlefield plans but suggested confidence that Washington and Ankara would get past the impasse.

“But we will work it out ... War sometimes doesn’t give you all good options. That’s the nature of war. It’s not a good situation,” he said.

Turkish warplanes carried out air strikes against Kurdish YPG fighters in northeastern Syria and Iraq’s Sinjar region late last month, killing fewer than 30 fighters, according to one estimate.

It was an unprecedented bombardment of groups linked to the Kurdistan Workers’ Party (PKK), which is fighting an insurgency against Ankara in Turkey’s southeast.

Turkish President Tayyip Erdogan has been vocal about the issue and has said he would try to convince Trump that Turkish-backed forces should be used to retake Raqqa from Islamic State when the two meet in mid-May.

The U.S. alliance with Turkey has proven decisive in the battle against Islamic State in Syria, providing the coalition with access to Turkey’s Incirlik air base to wage strikes against the militants, officials say.