ISTANBUL: A car bomb on Sunday struck the Turkish city of Gaziantep, a major refugee hub near the Syrian border, killing at least two policemen and wounding 22 other people as the country reels from a succession of militant attacks.

On a day of violence dubbed “Black Sunday” by the press, three Turkish soldiers died in a separate attack in the province of Mardin to the east, in an ambush by Kurdish militants who have killed hundreds of security forces in a renewed insurgency since last year.

Four suspected terrorists were detained in Ankara on suspicion of planning attacks and other May 1 events were canceled elsewhere over security fears.

One policeman was killed when the car bomb went off outside the police headquarters in Gaziantep and 23 people were wounded, including 19 police, the office of Gaziantep regional governor Ali Yerlikaya said in a statement

One more policeman later died of his wounds in hospital. The governor’s office statement did not say which group could be to blame.

Turkish news reports said the investigation was focusing on the possibility the attack was carried out by Daesh.

Police arrested the father of a known Daesh member in the city, named as Ismail G., and were carrying out DNA tests to ascertain if the son was the perpetrator, the Hurriyet daily said.

One of the main cities of Turkey’s southeast, Gaziantep has a population of around 1.5 million and is an important center for refugees who have fled the war in neighboring Syria.

Meanwhile three Turkish soldiers were killed and 14 others wounded in the attack in Kurdish-dominated Mardin province to the east of Gaziantep carried out by Kurdistan Workers Party (PKK) militants, the army said.

The attack took place in the Nusaybin district of Mardin, where the army has been conducting a military operation backed by a curfew against the PKK, it said.

Also on Sunday, four people were wounded in the Turkish town of Kilis, just south of Gaziantep on the Syrian border, by rockets fired from a Daesh-controlled area of Syria, Anatolia said.