Turkey deployed ground forces across the border into northern Iraq on September 8 for the first time since 2011 to pursue Kurdish militants involved in attacks that killed 31 Turkish police in recent days.
The state-run Anadolu Agency said 150 Turkish troops had entered northern Iraq with the aim of "destroying" two dozen militants of the Kurdistan Workers Party (PKK) who escaped from Turkey over the border after carrying out one of the attacks.
The move runs counter to U.S. efforts to galvanize the region into battle against the Islamic State (IS) extremist group because Turkey is now hunting down some of the very fighters who are trying to take back territory seized by the militant Sunni Muslim group in Syria and Iraq.
Turkish authorities haven't given a specific time frame for the operation in northern Iraq, although one official said it was a "short-term" offensive to root out rebels. Turkish jets also carried out more air strikes against rebel camps in the region.
The cross-border operations came after escalating clashes between government and Kurdish forces and a wave of attacks by the PKK that killed at least 31 soldiers and police since September 6. At least 15 police were killed in a car bombing on September 8.
Turkey, a NATO ally, signed up to participate more actively in the U.S.-led coalition against IS in July. It has allowed U.S. warplanes and drones to stage air strikes against IS from Turkish air bases and its jets have struck IS targets near its border in Syria.
But the country's main focus has been its fight against the PKK, which has waged a decades-long insurgency for Kurdish autonomy. President Recep Tayyip Erdogan has said in comments reported by the Turkish media that IS is a lesser threat to Turkey than the PKK.
Turkey's ground and aerial incursions against the PKK pose a difficult situation for the United States and its allies. The Kurds are doing much of the fighting against IS across the border in Syria and in northern Iraq.
The PKK, the target of Turkey's incursion into northern Iraq, has fought alongside Iraqi Kurdish Peshmerga forces, notably in Shingal, a predominantly Kurdish town which was taken by IS during last year's blitz.
Nazmi Gur, a former Turkish legislator who is in charge of Turkey's pro-Kurdish party's foreign relations, said Turkey's ground incursion will hurt efforts to liberate northern Iraq and Syria from IS.
"Turkey could not have helped [IS] more if it had tried," he said.
The United States called for the Turkish government and PKK to return to the negotiating table.
"It is important for Turkey and the PKK to return to their process of reaching a peaceful solution," White House spokesman Josh Earnest said.
The violence has upended a 2013 cease-fire aimed at allowing a final peace deal to be negotiated.
Critics accuse Erdogan of reigniting the fighting with the Kurds, after more than two years of peace efforts, for electoral gains.
Opponents say he aims to rally nationalist votes around his ruling Justice and Development Party and discredit a pro-Kurdish party whose electoral gains in June deprived Erdogan's party of securing a parliamentary majority. Erdogan denies the accusation.