Turkey has been enraged by America's support for the YPG, which Ankara sees as a terrorist organisation. However, Washington says it's backing the YPG in the fight against the Islamic State militant group in Syria.
Last month, Turkey launched a military incursion -- dubbed "Operation Olive Branch" -- into the Kurdish-held Afrin region of Syria in a bid to sweep the YPG from its southern border. It has also threatened to expand this offensive to the town of Manbij -- which is under the control of a YPG-led force -- and warned American troops stationed there to leave the area, something which the US rejected.
Their relations were also strained by what Ankara perceived to be a lack of solidarity on the American side following the failed 2016 coup in Turkey, particularly by Washington's refusal to extradite a US-based cleric that it blamed for ordering the rebellion.