The Turksih military added that they retaliated against attacks carried out from areas controlled by the YPG in Syria, targeting two military outposts in the Turkish city of Hatay.
The fighting erupted between Kurdish YPG militants and Turkish soldiers near Darbasiyah, a border town in Syria's Hassakeh province after Ankara's bombing of a Kurdish base a day earlier.
Ankara notified the United States and global coalition of plans to strike Kurdish-held areas in Syria and Iraq "weeks" in advance, the Turkish foreign minister said.
With less than an hour's notice to the U.S. -led coalition, Turkey, a U.S. North Atlantic Treaty Organisation ally, conducted airstrikes in northern Syria on Monday night, killing "a significant number" of Kurdish YPG fighters, who have been "very important to the fight against ISIS", Air Force Col. John Dorrian, the spokesperson for Operation Inherent Resolve, told reporters at the Pentagon on Wednesday.
Both areas were struck by Turkish warplanes on Tuesday in attacks that drew criticism from Washington and led to the deaths of up to six Iraqi Peshmerga - troops belonging to the Ankara-allied Kurdish Regional Government.
Both Turkey and the US have designated the Kurdistan Workers' Party or PKK, a militant group operating in northern Syria and Iraq, as a terrorist group. The YPG is a close U.S. ally against Isis but is seen by Ankara as a terrorist group because of its ties to Turkey's Kurdish rebels.
The U.S.is especially concerned that Turkey gave little advance warning to the U.S. Combined Air Operations Center in Baghdad and described only a general area in which they would take place, Dorrian said.
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But State Department spokesman Mark Toner said "these kinds of actions frankly harm the coalition's efforts" against IS.
"Twenty of our fighters were killed and 18 more injured as a result of the Turkish airstrikes", Redur Khalil, the spokesperson for the YPG said.
To the irritation of the US, Turkey carried out airstrikes Tuesday against US allies in Syria and Iraq.
Turkey responded by firing a barrage of artillery at the YPG position, with Kurdish fighters returning fire with rockets, according to the Syrian Observatory for Human Rights. "We notified the U.S. and Russian Federation within two hours before the operation, in accordance with our agreements", Cavusoglu said. They shared posters saying "every dead Kurd is a victory for ISIS" and a video saying that "Turkey is saving ISIS by attacking the Kurds".
Dorrian said Turkey gave the USA less than an hour's notice, and declined to be more specific.
The YPG is receiving training, equipment and air support from the U.S., and is advancing on the ISIL stronghold of Raqqa. And how does the USA expect the Kurds to take over Raqqa once it's "liberated" when the Turks keep killing them? The SDF also includes Arab fighters.