A suicide bombing on Istanbul's main shopping street on Saturday killed five people, including the assailant, leaving Turkey reeling from the second such attack within a week, the latest hitting a popular tourist area.
Among the injured in the blast in Turkey's largest city and economic hub were 24 foreigners, the local government's office said.
Health Minister Mehmet Muezzinoglu said at least 36 people were wounded in the explosion near a shopping area on Istiklal boulevard, a busy thoroughfare in the centre of Istanbul just off Taksim Square.
Two of those killed in the blast held joint US-Israeli citizenship.
"Turkey has once again suffered a horrific terrorist attack, and we remain steadfast in our support for our NATO Ally and partner," White House National Security Council spokesman Ned Price said in a statement.
"These repeated acts of terrorism in Turkey must come to an end."
Israeli Prime Minister Benjamin Netanyahu, confirming the two deaths, said it was "still unclear whether the attack targeted Israelis."
Israeli media reported several Israelis were injured, including two critically, while Turkey's Dogan news agency reported that at least one of the dead was Israeli and another Iranian. Seven people were listed in serious condition.
The Irish government confirmed that a number of Irish citizens were injured, the Press Association reported.
UN Secretary General Ban Ki-moon condemned the attack, saying through his spokesman that the "perpetrators of this appalling violence must be brought tojustice, consistent with human rights obligations."
Prime Minister Ahmet Davutoglu was quoted by Dogan as saying investigations would soon be completed and shed light on the incident.
Police are testing the DNA of the family member of a suspect, while Turkish authorities were implying Islamic State militants were to blame for the blast, Dogan reported.
No group has so far claimed responsibility for the attack.
Istiklal street was largely shut down by police following the attack just before 11 am (0900 GMT) and remained that way into the evening.
A video from a closed circuit camera emerged showing the moment of the explosion which appeared to go off in the middle of a large group of people near a popular fish market.
"I had just ordered a coffee in the nearby shopping mall when we heard the explosion. It actually was not so loud. At first we thought a large piece of furniture had fallen down from a height," witness Tolsum Merey said.
The blast comes as Turkey is on edge following a massive car bombing in Ankara on March 13 that killed 37 people.
That attack was claimed by the Kurdistan Freedom Falcons (TAK), a splinter group of the banned Kurdistan Workers' Party (PKK).
A similar attack last month in the capital left 30 dead and was also claimed by TAK.
The US embassy issued a warning about a potential terrorist act before the Ankara bombing occurred.
German diplomatic missions and schools were closed this week in Istanbul and Ankara following what was described as a "concrete" terror threat, apparently from Islamic State. Authorities said the embassy in Ankara and consulate in Istanbul would reopen on Monday.
Following the violence and warnings, Istanbul's normally bustling night life was relatively quiet, with many bars and restaurants in the Taksim area seeing few costumers.
The combined effect of the attacks as well as the war in neighbouring Syria and a diplomatic row with Russia is having a devastating impact on Turkey's vital tourism sector.
Violence has been spiralling in Turkey since last summer. The largest terrorist attack in the country's history came in October, when 100 people were killed in a series of bombings in Ankara blamed on Islamic State.
Meanwhile, the country has seen the conflict with PKK militants from the Kurdish minority reignited after a two-year ceasefire collapsed in July and peace talks were abandoned.
Hundreds have died, including civilians, in the mostly Kurdish south-eastern regions of the country in recent months. Kurds have long complained of systemic discrimination.
Turkey launched fresh airstrikes against the PKK's bases in northern Iraq on Friday and Saturday, Turkish and Kurdish media outlets said.