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."
Germany had shut its diplomatic missions and schools on Thursday, citing a specific threat. U.S. and other 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, declining to be named as the investigation is ongoing.
Another official said investigations were focusing on three possible suspects, all of them male and two of them from the southern city of Gaziantep near the Syrian border. There was no further confirmation of this.
Armed police sealed off the shopping street where half a dozen ambulances gathered. Forensic teams in white suits searched for evidence as police helicopters buzzed overhead.
"I saw a body on the street. No one was treating him but then I saw someone who appeared to be a regular citizen trying to do something to the body. That was enough for me and I turned and went back," one resident told Reuters.
Istiklal Street, usually thronged with shoppers at weekends, was quieter than normal as more people are staying home after a series of 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 latest attack brought widespread condemnation.
Iranian Foreign Minister Mohammad Javad Zarif, on an official visit to Istanbul, said it showed "the ugly face of terrorism". France condemned it as "despicable and cowardly".
NATO chief Jens Stoltenberg described it as "another terrorist outrage against innocent civilians", while the U.S. State Department said it was the latest "indefensible violence targeting innocent people throughout Turkey".
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.
A 2-1/2-year PKK ceasefire collapsed last July, triggering the worst violence in the southeast since the 1990s. Hundreds have since died.
Separately, a police officer and a soldier died in clashes with militants in the southeastern city of Nusaybin, security sources said.
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.
At the height of the PKK insurgency in the 1990s, the Newroz festival often saw clashes between Kurdish protesters and security forces.
(By Nick Tattersall and Ayla Jean Yackley; Additional reporting by Orhan Coskun, Asli Kandemir, Humeyra Pamuk, Daren Butler, Parisa Hafezi in Turkey, John Irish in Paris, Maayan Lubell in Jerusalem, Hans-Edzard Busemann in Berlin and Idrees Ali in Washington; Padraic Halpin in Dublin; writing by David Dolan and Nick Tattersall; editing by David Clarke)