A suicide bomb attack in a busy shopping district in Turkey's Istanbul killed four people on Saturday.
At least 36 people were wounded in the blast, which took the death toll from suicide attacks in Turkey to 80 this year.
NATO member Turkey has been targeted by Kurdish and Islamic State militants in recent months.
The blast likely targeted the country's security personnel as it took place near a place where police buses are routinely parked, in Istiklal Street in Istanbul.
"People didn't know what was going on. It was very chaotic. Everyone was screaming and running away. Istanbul is on high alert and people are very worried as to what will happen next," a witness told BBC.
Turkish Prime Minister Ahmet Davutoglu said the country will not back away from fighting the "centers of terrorism" in the face of increasing attacks on its soil.
Eleven Israelis, two Irish citizens and person each from Germany, Iceland, United Arab Emirates and Iran were also among the injured.
"There is information that it is an attack carried out by an ISIS member, but this is preliminary information, we are still checking it," Israeli Prime Minister Benjamin Netanyahu said, according to Reuters.
Earlier this week a car bomb exploded in capital Ankara killing 37 people and wounding more than 100.
Turkey said a 24-year-old woman affiliated with the Kurdish militias was the suicide bomber.
In February, an attack on a military bus in the city centre had killed 28 people. The bomb attack was claimed by the Kurdistan Freedom Hawks (TAK), a Kurdish militant group affiliated to the PKK.
"No center of terrorism will reach its aim with such monstrous attacks ... Our struggle will continue with the same resolution and determination until terrorism ends completely," Davutoglu said in a statement after Saturday's attack.