A suicide bomber killed four people on Saturday in a busy shopping district in the heart of Istanbul, pushing the death toll from four separate suicide attacks in Turkey this year to more than 80.
Prime Minister Ahmet Davutoglu said the blast was "inhumane" and would not stop Turkey, which has been targeted by Kurdish and Islamic State militants, from fighting "centers of terrorism".
Israel said two of its citizens died in the attack, Washington said two Americans had been killed and a Turkish official said one victim was Iranian, suggesting that some of the dead may have had dual nationality.
The blast, which also wounded at least 36 people, was a few hundred meters from an area where police buses are often stationed. It sent panicked shoppers scurrying into alleys off Istiklal Street, a long pedestrian avenue lined with international stores and foreign consulates.
"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 told reporters, using another name for Islamic State.
He said a third Israeli may have died. Israel also said 11 of its citizens had been wounded while Ireland said "a number" of Irish were hurt.
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 neighboring Syria.
Turkey is battling a widening Kurdish insurgency in its southeast, which it sees as fueled by the territorial gains of Kurdish militia fighters in northern Syria, and has also blamed some of the recent bombings on Islamic State militants who crossed from its southern neighbor.
"No center of terrorism will reach its aim with such monstrous attacks," Davutoglu said in a written statement. "Our struggle will continue with the same resolution and determination until terrorism ends completely."