Turkey has summoned the US ambassador after a US State Department spokesman said Washington did not regard Syria's Kurdish Democratic Union Party (PYD) as a terrorist organisation, a Turkish foreign ministry official said.
A Turkish foreign ministry official said US Ambassador John Bass was called to the ministry on Tuesday where a senior Turkish official expressed Ankara's concerns.
The official spoke on condition of anonymity because she was not authorised to make public statements.
Ankara sees the PYD as a sister arm of the Kurdistan Workers Party (PKK), which is fighting for Kurdish autonomy in southeastern Turkey and is classed as a terrorist organisation by Turkey, the US and the European Union.
Al Jazeera's Rosiland Jordan, reporting from Washington DC, said the US supports the PYD in its struggle against the Islamic State of Iraq and the Levant (ISIL) group in Syria.
Turkish President Recep Tayyip Erdogan expressed anger on Sunday after a delegation including senior US diplomat Brett McGurk, special envoy to an international coalition fighting ISIL, met Kurdish fighters in Kobane.
Asked about the difference of opinion between the two NATO allies on Monday, State Department Spokesman John Kirby said: "This is not a new concern, as I said, that the Turks have proffered. And we don't, as you know, recognise the PYD as a terrorist organisation.
"We recognise that the Turks do, and I understand that. Even the best of friends aren't going to agree on everything. Kurdish fighters have been some of the most successful in going after Daesh [ISIL] inside Syria."
Source: Al Jazeera and agencies