Kurdish militants claimed the murder of two Turkish police officers as revenge for a suicide bombing, blamed on Islamic State jihadists, that killed 32 activists near the Syrian border.
The attack by Kurdistan Workers' Party (PKK) militants in the town of Ceylanpinar intensified fears that the fighting raging in Syria between Kurds and IS extremists is spilling over onto Turkish territory.
In Ankara, Prime Minister Ahmet Davutoglu convened his cabinet to discuss an action plan for strengthening security on Turkey's border with Syria after Monday's devastating attack in the town of Suruc.
Officials said the suicide bomber had been identified as a 20-year-old Turkish student who had been in contact with IS jihadists for several months.
The attack has fanned fears that IS, which has established a self-declared "caliphate" across parts of Syria and Iraq, are extending their reach and prompted US President Barack Obama to discuss improving regional security with his Turkish counterpart Recep Tayyip Erdogan.
The authorities had earlier appeared at a loss to explain the deaths of the police officers, who were shot in the head at their shared home in an apartment block in southeastern town of Ceylanpinar near the border.
But the military wing of the PKK -- outlawed as a terrorist group by Turkey and its Western allies -- said the attack was a reprisal.
"A punitive action was carried out... in revenge for the massacre in Suruc," the People's Defence Forces (HPG) said in a statement on its website, accusing the two officers of cooperating with IS.
It described the attackers as an "Apoist team of self-sacrifice," in reference to the PKK's jailed leader Abdullah Ocalan, whose nickname is "Apo" or "uncle".
- Students killed -
The victims of the Suruc attack on Monday were activists, many of them students and the youngest aged just 18, who had gone there to prepare an aid mission for the Syrian town of Kobane just over the border.
A Turkish official told AFP that DNA testing had confirmed a 20-year-old Turkish man reportedly linked to IS militants carried out the suicide bombing.
Media reports named the bomber as university student Seyh Abdurrahman Alagoz from Adiyaman in southeastern Turkey.
His father, Zeynel Abidin Alagoz, told the Dogan news agency that Abdurrahman had gone missing with his elder brother Yunus Emre six months ago and he had reported this to police three months ago.
"My son was very young, he would not hurt a fly," the father told Dogan.
The Suruc strike was the first time the Turkish government directly blamed IS for a strike inside the country.
Signup to our newsletter and follow us on Facebook and Twitter!
Turkish authorities are also investigating whether there is any connection between the Suruc assault and an attack in the Kurdish majority city of Diyarbakir last month that killed four people.
Turkey has long been accused by its Western partners of failing to properly control its 911-kilometre (566-mile) frontier with Syria and even of colluding with IS, allegations it fiercely denies.
Deputy Prime Minister Bulent Arinc said ministers in the cabinet meeting had discussed implementing an "integrated system" to secure the border with war-torn Syria "to counter the threat of IS".
Hours later, Erdogan spoke on the phone with Obama about how to better secure Turkey's borders and stem the flow of foreign fighters into Iraq and Syria.
They discussed "deepening our ongoing cooperation in the fight against ISIL and common efforts to bring security and stability to Iraq and a political settlement to the conflict in Syria," the White House said in a statement, using an acronym for the IS group.
- Twitter blocked -
The unrest comes at a critical time for Turkey following elections in which the ruling Justice and Development Party (AKP) of President Recep Tayyip Erdogan lost its overall majority in parliament for the first time since coming to power in 2002.
Tensions are running high and the pro-Kurdish Peoples' Democratic Party (HDP) has called for a major "rally for peace" on Sunday afternoon in Istanbul.
Pro-Kurdish demonstrations in Istanbul over the last two days have been dispersed by police using water cannon and tear gas.
At a funeral procession for three of the victims of Monday's bombing in the Gazi district of Istanbul, left-wing masked gunmen accompanied the coffins, an AFP photographer said.
Max Abrahms, professor of political science at Northeastern University and a member at the US Council on Foreign Relations think-tank, told AFP the PKK attack was a message for the Turkish government.
"The violence is intended to convey that Turkish officials are at risk if they continue to aid Islamic State at the expense of the Kurds."
Meanwhile, a Turkish court ordered a ban on publishing images of the deadly suicide bombing in Suruc on all print, visual and online media, including Twitter.
Turkish users had earlier reported problems in accessing Twitter after the court order. By 4:00 pm (1300 GMT), access had been restored.
"We notified Twitter about 107 URL addresses that must be removed in compliance with the court order," a government official told AFP, saying that Twitter blocked those addresses after talks with the government.