Spokesman Mark Toner was speaking after Turkish warplanes bombed locations in north-eastern Syria and northern Iraq, killing not just PKK fighters but also Kurdish members of the Syrian Democratic Forces (SDF), an anti-ISIS group backed by the U.S.
Dorrian spoke hours after the Turkish embassy in Washington issued a statement from the country's Foreign Ministry detailing what it said were the steps Turkey took to make the United States and other countries active in Syria aware of its plans, saying the United States and Russian Federation were "duly informed through both military and diplomatic channels".
Speaking to reporters at the Pentagon, Dorrian said the U.S.is concerned that Turkey didn't coordinate the strikes with the USA -led coalition against the Islamic State group.
"Our partner forces have been killed by Turkey strike, they have made many sacrifices to defeat ISIS", the American Colonel said.
"At least six people were killed, five from the Peshmerga and a sixth from Asayish (Rojava)", Lieutenant General Jabbar Yawar, secretary-general of the Peshmerga ministry in Iraq's autonomous Kurdish government, told AFP.
Syrian Democratic Council co-president Ilham Ahmad said local SDF commanders had asked the United States to prevent Turkish and Syrian jets flying over Rojava, the Kurdish-controlled zone of northern Syria.
The Turkish military carried out airstrikes Tuesday in Iraq and Syria, saying it was targeting the Kurdistan Workers Party, or PKK, a group it considers to be terrorist. "We need to be one hand against our enemy", he said.
Mr. Toner told reporters Tuesday that Ankara's need to crack down on the PKK and its affiliates, which are responsible for a number of terror attacks inside Turkey, is not lost on US officials. An attempted Turkey-PKK peace process and ceasefire broke down in July 2015.More news: Senators head to "commander in chief's house" for North Korea briefing
More news: Coroner says autopsy of dead girl inconclusive
More news: US Senate Confirms Trump Nominee Rod Rosenstein for Deputy Attorney General
The PKK established a presence in the Sinjar region during the summer of 2014 when its fighters helped tens of thousands of Yazidis escape Mount Sinjar, which was encircled by IS militants at the time.
Yesterday, Turkey claimed YPG forces in the north-west Syrian enclave of Afrin had attacked two of its military border posts in Hatay province with mortar fire.
The strikes underscore the delicate political tightrope the United States is treading in Syria - and to a lesser extent in Iraq - where it is relying heavily on Kurdish forces to conduct the ground fight against the Islamic State group. Turkey says they're an extension of the Workers' Party and that its fighters are finding sanctuaries in Iraq and Syria. It said the operations were conducted to prevent infiltration of Kurdish rebels, weapons, ammunition and explosives from those areas into Turkey.
There were no casualties in any of the attacks, the army said.
YPG spokesman Redur Xelil said 20 YPG fighters were killed and 18 were wounded, three of them critically.
"We were troubled by that", Air Force Col. John Dorrian said of Turkey's failure to give adequate warning of airstrikes Tuesday in the Hasaka area.
The PKK is listed as a terrorist organisation by Turkey, the United States, and European Union, and it resumed its armed campaign against Turkey in July 2015.