By Nick Tattersall and Ayla Jean Yackley
A suicide bomber has killed two Israelis and two other people in a busy shopping district in the heart of Istanbul, the fourth such attack in Turkey so far this year.
Prime Minister Ahmet Davutoglu said Saturday's blast was "inhumane" and would not stop Turkey, which has been targeted by Kurdish militants and Islamic State in recent months, from fighting "centres of terrorism".
The blast, which wounded at least 36 people, was a few hundred metres from an area where police buses are often stationed and sent panicked shoppers scurrying into alleys off Istiklal Street, a pedestrian avenue lined with international stores and foreign consulates.
The explosion killed at least two Israeli citizens, Prime Minister Benjamin Netanyahu said, adding that a third may have died. Israel said earlier that 11 of its citizens were injured while Turkish officials said one Iranian was also among the dead. Ireland said "a number" of Irish were hurt.
"There is information that it is an attack carried out by an ISIS member, but this is preliminary information, we are still checking it." Netanyahu told reporters, referring to Islamic State.
The attack will raise further questions about the ability of NATO member Turkey to protect itself against a spillover of violence from the war in neighbouring Syria.
US, Australian and European embassies had warned their citizens to be vigilant ahead of Newroz celebrations this weekend, a spring festival largely marked by Kurds that has turned violent in the past.
There was no immediate claim of responsibility. Two senior officials said the attack could have been carried out by the Kurdistan Workers Party (PKK), fighting for Kurdish autonomy in the southeast, or by an Islamic State militant.
A PKK offshoot claimed responsibility for two suicide bombings in the capital Ankara over the past month which killed 66 people. Islamic State was blamed for a suicide bombing in Istanbul in January which killed at least 12 German tourists.
One of the officials said Saturday's bomber, who also died in the blast, had planned to hit a more crowded location but was deterred by the police presence.
"The attacker detonated the bomb before reaching the target point because they were scared of the police," the official said.
Istiklal Street, usually thronged with shoppers at weekends, was quieter than normal as more people are staying home after the recent deadly bombings.
Health Minister Mehmet Muezzinoglu said 36 people had been wounded, seven of them in serious condition. At least 24 of the wounded were foreigners, according to Istanbul's governor.
Turkey is still in shock from a suicide car bombing last Sunday at a crowded transport hub in the capital Ankara which killed 37 people and a similar bombing in Ankara last month in which 29 died. A PKK offshoot claimed responsibility for both.
The Kurdish-rooted opposition Peoples' Democratic Party (HDP) condemned the bombing. The PKK's umbrella group said it opposed targeting civilians and condemned attacks on them.
In its armed campaign in Turkey, the PKK has historically struck directly at the security forces but recent bombings suggest it could be shifting tactics.