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The US has urged Turkey to not "slaughter" Kurds in Syria, Secretary of State Mike Pompeo said Thursday, as American troops prepare to withdraw.

Pompeo said he was seeking to reassure Washington's Kurdish allies in northern Syria, following President Donald Trump's abrupt decision to withdraw troops from the war-torn country.

"The importance of ensuring that the Turks don't slaughter the Kurds, the protection of religious minorities there in Syria. All of those things are still part of the American mission set," Pompeo told Newsmax, a US news and opinion site popular with conservatives.

The decision led Defence Secretary Jim Mattis to resign, angering the president who said later that he would slow down the planned exit.

Pompeo insisted that the withdrawal would go ahead but would not give a timeline. Turkey has vowed to rid Syria of the Kurdish People's Protection Units (YPG), which he sees as linked to the Kurdish Workers' Party (PKK).

The two sides have fought a bitter war since 1984, leaving thousands dead.

Kurdish fighters make up the rump of the Syrian Democratic Forces, who with Washington's backing have battled the Islamic State group and seized about a quarter of Syria.

Syria's regime said Wednesday that YPG fighters have left Manbij, a key city seized from IS near the Turkish border, in light of the planned US pullout.

Trump, who declared that US troops were no longer needed as IS was defeated, spoke to Erdogan before his troop decision.

"There are lots of places where we need to work with President Erdogan and the Turkish leadership to get good outcomes for the United States," Pompeo said.

National security adviser John Bolton and the US pointman on Syria, Jim Jeffrey, are both due to hold talks next week in Turkey.