The United States has expressed "deep concern" over air strikes by Turkey and says they were not authorised by the US-led coalition fighting Islamic State.
The Turkish planes bombed Kurdish fighters in Iraq's Sinjar region and northeast Syria on Tuesday, killing at least 20 in a campaign against groups linked to the outlawed Kurdistan Workers Party.
"We have expressed those concerns with the government of Turkey directly. 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" Islamic State, State Department spokesman Mark Toner said in a conference call with reporters.
A Turkish military statement said around 70 militants were killed in Tuesday's operations inside the two neighbouring states.
The Turkish raids showed the challenges facing US-led attempts to defeat Islamic State in Syria and risk increasing tension between NATO allies Washington and Ankara over Kurdish combatants who have been crucial in driving back the jihadists.
The Turkish military said the two regions it struck had become "terror hubs" and the aim of the bombardment was to prevent the PKK sending weapons and explosives for attacks inside Turkey.
Designated a terrorist group by Turkey, the United States and the European Union, the PKK has waged a three-decade insurgency against the Turkish state for Kurdish autonomy. More than 40,000 people have been killed in the conflict, most of them Kurds.
Turkish security sources said 13 PKK militants had been killed on Tuesday in operations backed by the air force in the largely Kurdish southeast of Turkey. Two Turkish soldiers were also killed when a roadside handmade bomb planted by the PKK blew up in Sirnak province.
Turkey has regularly bombed the mountainous border area between Iraq and Turkey where PKK militants are based since a ceasefire broke down in July 2015. But Tuesday's raid was the first time Turkish forces have targeted its affiliate in the northwestern Sinjar area.