WASHINGTONWASHINGTON (Reuters) - The Pentagon acknowledged on Monday that a Turkish offensive against a U.S.-backed militia in Syria had affected the fight against Islamic State and led to an "operational pause" in eastern Syria.

The United States and Turkey, while themselves allies in the North Atlantic Treaty Organization, have diverging interests in the Syrian civil war, with Washington focused on defeating Islamic State and Ankara keen to prevent Syria’s Kurds from gaining autonomy and fuelling Kurdish insurgents on its soil.

Turkey launched an offensive on Afrin in January to drive out the Kurdish YPG militia, which it regards as a terrorist group and as an extension of a Kurdish insurgency at home.

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Pentagon spokesman Colonel Robert Manning told reporters that the pause meant that some ground operations by the Syrian Democratic Forces (SDF), a U.S.-backed umbrella group dominated by the YPG, had been temporarily put on hold. Manning said air strikes by the U.S.-led coalition fighting Islamic State had not been affected and the SDF continue to hold territory taken back from the militant group.

While much of the fighting against Islamic State has moved to small pockets, the United States still needs the YPG to hold territory, ensuring that the Islamist militant group does not re-emerge.

Major Adrian Rankine-Galloway, another Pentagon spokesman, said the U.S. military had seen SDF fighters leave the fight against Islamic State.

"Some fighters operating within the SDF have decided to leave operations in the middle Euphrates river valley to fight elsewhere, possibly in Afrin," Galloway said.

A U.S. official, speaking on condition of anonymity, said hundreds of SDF fighters had left the middle Euphrates river valley in the past two weeks.

Turkey has been enraged by U.S. support for the YPG, which Ankara views as a terrorist organization. Washington has backed the YPG in the fight against Islamic State in Syria.

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Last week, U.S. Army General Joseph Votel, the head of U.S. Central Command, said Turkish activity in Afrin was distracting from the fight against Islamic State.

The United Nations Security Council passed a resolution on Feb. 24 demanding a full month-long ceasefire across Syria excepting only groups that it had designated as terrorists.

Turkey has said the ceasefire resolution does not apply to the YPG and has rejected Western calls for it to implement the truce.