The PKK terrorist group's Syrian offshoot the Democratic Union Party (PYD) will not participate in the Russian-sponsored Syrian National Congress in Sochi, diplomatic sources said Thursday.

The sources stressed that Kurdish people must be represented in the congress, but not by terrorists, adding that no trace of the Assad regime should be left in the new Syrian administration.

The foreign ministry sources also noted that the U.S. did not keep its promise regarding not arming the PYD and its armed wing People's Protection Units (YPG), which dominate the Syrian Democratic Forces (SDF) backed by Washington in fight against Daesh terrorist group.

"Information sharing is not enough, the U.S. must be clear about when [arming of PYD/YPG] will end," the sources said.

They added that different statements come from the U.S., but for Turkey even "one-hour" cooperation with PYD/YPG terrorists is harmful and worrying.

Ankara strongly disagrees with Washington's backing the YPG in the war against Daesh in Syria and the issue is one of the main reasons underlying the strained ties between the two NATO allies. While the U.S. insists that supporting the YPG was the only option to defeat Daesh, Ankara says an alternative could be found with local Arab tribes backed by countries in the region instead of supporting and arming one terrorist group against another. Ankara also had said the two countries can carry out a joint operation in Raqqa, before the YPG-led offensive started, but Washington rebuffed the offer.

The PKK is listed as a terrorist organization in Turkey, the U.S. and the EU. More than 40,000 people have been killed in its over three-decade terrorist campaign against the Turkish state.