"That Tillerson eschewed this sort of support in what he knew would be a tense and critical meeting with President Erdogan smacks of either poor staff work or unsafe naïvete on his part", Kirby added.
Speaking with reporters on the sidelines of the Brussels Nato meeting on Wednesday, Secretary of Defence Jim Mattis said talks with his Turkish counterpart had been open, but hard.
Regarded by Ankara as a terrorist organization and the Syrian branch of the outlawed Kurdistan Workers' Party (PKK), the YPG forms the largest part of the SDF, a US-backed anti-Damascus militant group composed of mainly Kurdish forces.
"You hit us, we will respond aggressively. We continue to collaborate on ways to ensure their legitimate concerns are addressed".
The meeting came on the evening of a visit to Turkey by US Secretary of State Rex Tillerson.
Islamic State fighters were driven a year ago from all the population centres they occupied in both Syria and Iraq, but Washington still considers them a threat, capable of carrying out an insurgency and plotting attacks elsewhere.
Washington explains its backing of the Kurds by the need to fight against the Daesh terrorist group, outlawed in the United States and Russian Federation among other states.
Narrow DACA Deal Fails In Senate As Immigration Votes Begin
But some lawmakers oppose anything that might be seen as rewarding the parents for entering the country illegally. It would also place some limitations on Dreamers' ability to sponsor extended family members for visas.
In January the USA pledged continued support for the YPG and SDF to ensure the jihadi militants do not form a resurgent movement, a strategy which further enraged Turkey in the process.
A State Department spokesman traveling with Tillerson said he and Erdogan "engaged in a productive and open conversation about a mutually beneficial way forward in the US-Turkey relationship".
Turkish officials alleged the staff members had ties to cleric Fethullah Gulen, whom Ankara accuses of being responsible for a deadly failed coup in 2016. The United States convicted a Turkish banker mast month for violating sanctions on Iran in a case that included testimony alleging corruption by top Turkish officials.
Erdogan has accused the USA of "building an army of terror" on his border and, in extraordinary language coming from a fellow North Atlantic Treaty Organisation ally, threatened to "drown" US-backed forces in Syria.
Last year, the two countries temporarily suspended non-immigrant visa services at their respective embassies over the arrest of several staff members at the United States consulate in Istanbul.
The two officials reaffirmed the longstanding defense relationship between the United States and Turkey.
Beyond the possibility of a clash between Turkey and US-backed forces, if not USA troops themselves, United States officials are concerned that Turkey's move to attack the Kurds' enclave of Afrin in northwest Syria is potentially undermining the coalition fight against ISIS, as YPG fighters helping the U.S. rout ISIS in eastern Syria have begun leaving to help their compatriots in Afrin.