A former commander of United States' forces in Europe advised U.S. President Joe Biden on Thursday to end assistance for the PKK terrorist group's Syrian branch YPG.

Speaking at a virtual discussion with the Hudson Institute's Senior Fellow Michael Doran, Ben Hodges called U.S. support for the group in Syria an "irritant" for Turkey.

"Even those who are not here Erdoğan supporters, which is about half the country, are really annoyed about this YPG support," said the former commander, referring to President Recep Tayyip Erdoğan.

He also urged Biden to visit Erdoğan in Ankara within the next three months.

The greatest challenge that Turkey-U.S. relations face is not Ankara’s purchase of the Russian S-400 missile defense system but rather Washington’s support for the YPG, Defense Minister Hulusi Akar stated last year.

The PKK is a designated terrorist organization in the U.S., Turkey and the European Union, and Washington's support for its Syrian affiliate has been a major strain on bilateral relations with Ankara.

The U.S. primarily partnered with the YPG in northeastern Syria in its fight against the Daesh terrorist group, while Turkey has strongly opposed any terrorist presence in the region. Ankara has long objected to the U.S.' support for the YPG, a group that poses a threat to Turkey and that terrorizes local people, destroying their homes and forcing them to flee.

Under the pretext of fighting Daesh, the U.S. has provided military training and given truckloads of military support to the YPG, despite its NATO ally's security concerns. Underlining that one cannot support one terrorist group to defeat another, Turkey conducted its own counterterrorism operations, over the course of which it has managed to remove a significant number of terrorists from the region.