ANKARA (Reuters) - U.S. Defense Secretary Ash Carter said he would stress the need to respect Iraq’s sovereignty during a visit on Friday to Turkey, which has been locked in a dispute with Baghdad over who should participate in the campaign to retake Mosul from Islamic State.

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Carter, acknowledging it was a delicate issue, declined to explicitly say whether he thought Turkey should be allowed toparticipate in the operations in Iraq. Washington in the past has deferred that matter to Baghdad.

“Of course we’ll talk about that. And yes, of course there are sensitivities there. We conduct ourselves, and the coalition does, respecting Iraqi sovereignty. That’s an important principle of ours,” Carter said.

Turkish President Tayyip Erdogan has been frustrated that NATO member Turkey has not been more involved in the U.S.-backed assault on Mosul and angered by Washington’s support for Kurdish militia fighters battling Islamic State in Syria.

Asked about Turkish air strikes that pounded a group of Kurdish fighters allied to a U.S.-backed militia in northernSyria, Carter said he was not certain about what precisely transpired.

“I can’t clarify that now,” he said.

A U.S. defense official said on Thursday the specific groups struck by Turkish jets were not themselves U.S.-backed, but were “close to and friendly with” the fighters Washington is working with.

Ankara has been in a row with Iraq over the presence of Turkish troops at the Bashiqa camp near Mosul, as well as over who should take part in the offensive in the largely Sunni Muslim city of Mosul, once part of the Ottoman empire and still seen by Turkey as firmly within its sphere of influence.

Erdogan has warned of sectarian bloodshed if the Iraqi army relies on Shi’ite militia fighters.

A U.S. defense official, speaking on condition of anonymity, acknowledged Turkey had legitimate security concerns in Iraq but added that Washington had been clear that “no military should be operating in areas (of Iraq) where they aren’t invited expressly.”

“We have been working behind the scenes to get the Iraqis and the Turks to come to an understanding about how we’re going to move forward on Mosul,” the official said.

Carter steered clear of directly commenting on the matter ahead of his talks in Turkey. He acknowledged the United States was partnering with both Iraq and Turkey in the fight against Islamic State.

“These are two close friends of ours. In the case of Turkey, it’s a NATO ally. And we want to keep everybody focused on the objective here, which is to defeat ISIL, because that is a threat to all three of us,” Carter said.