The United States said it is "deeply concerned" that Turkey conducted airstrikes in Syria and Iraq without "proper coordination" with Washington or the coalition forces, media reports said.
"We have expressed those concerns to the government of Turkey directly," State Department spokesman Mark Toner told a regular news briefing on Tuesday. "These air strikes were not approved by the coalition and led to the unfortunate loss of life of our partner forces in the fight against ISIS (the Islamic State) that includes members of the Kurdish Peshmerga."
The Turkish warplanes on Tuesday conducted airstrikes targeting the positions of the Kurdistan Workers' Party (PKK) in northern Iraq, killing at least five Iraqi Kurdish fighters and wounding ten others, Xinhua news agency cited local Kurdish media reports.
The Turkish jets, in the early hours of the day, hit the PKK positions in different areas in northwestern Iraq, but the Iraqi Kurdish fighters who were close to the PKK positions apparently were killed by mistake.
Meanwhile, the Turkish military said it has carried out airstrikes against suspected Kurdish rebel positions in northern Iraq and in northeastern Syria, in a bid to prevent militants from smuggling fighters and weapons into Turkey.
Toner also noted that the Iraqi government had expressed concerns with Turkey's airstrikes, stressing that "military action in Iraq should respect Iraqi sovereignty".
The PKK, which wages a separatist war against Turkey since 1984, is listed as a terrorist organisation by Turkey, the US and the European Union.