A suicide blast ripped through a top shopping district in Istanbul, killing at least four people and injuring dozens, the second attack in the heart of Turkey's major cities in a week.
The sixth major bombing since July targeted Istiklal Caddesi, a bustling pedestrian street usually thronged with shoppers, tourists and buskers but which was still relatively quiet when the bomber struck at around 11:00 am (0900 GMT).
Turkish media said three Israelis and one Iranian were killed, while two of the Israelis were also named by Washington as US citizens.
CCTV footage circulated by Turkish media showed the alleged bomber wearing a long coat approaching a small group of people outside a local government building directly before the blast.
Istanbul governor Vasip Sahin said the building appeared to be the intended target, and a Western diplomatic source said the attacker may also have had tourists in his sights.
A waiter working in a nearby restaurant, who only gave his first name, Mustafa, told AFP he heard "a loud explosion".
"When I came out, people were running in every direction, but I ran towards the blast site to see what happened," he said, describing scenes of "carnage" with "people lying all about".
No group has claimed responsibility for the attack, but pro-government media blamed it on the Islamic State (IS) group, which has been accused of several attacks in Turkey including a deadly suicide bombing near the Blue Mosque in January.
Several media outlets named the alleged perpetrator of Saturday's attack as Savas Yildiz, a 33-year-old Turkish radical.
Turkish Prime Minister Ahmet Davutoglu vowed to track down those responsible, while NATO allies the US and Europe said they stood by Turkey.
"We will fight with determination and perseverance until all forms of terrorism are eradicated," Davutoglu said.
- 'Unqualified support' -
Deputy health minister Ahmet Baha Otuken named an Israeli woman, Simha Demri, as one of the victims.
The CNN-Turk channel said 31-year-old Iranian Ali Rıza Khalman was killed along with two other Israelis -- Yonathan Suher, 40 and Avraham Goldman, 70. They were also named as US citizens by the State Department.
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Of the 36 injured, Health Minister Mehmet Muezzinoglu said 12 were foreigners. Six were Israelis, two were Irish and the four others from Germany, Iceland, Iran and Dubai, his office said.
Turkey is on a knife-edge after five major bombings that have killed over 200 people since July, three of which have been blamed on IS while Kurdish groups have been accused of carrying out the others.
Embarrassed by accusations of security lapses, President Recep Tayyip Erdogan has lashed out at journalists, activists and intellectuals who criticise his renewed battle against the Kurds, calling them "accomplices" of terror.
Deputy prime minister Numan Kurtulmus told reporters he expected "unqualified support from those who call themselves Turkey's friends" -- apparently referencing recent criticism of Erdogan in Europe.
US State Department spokesman John Kirby condemned the attack, saying: "The United States stands in solidarity with our NATO ally Turkey in combating the common threat of terrorism."
United Nations chief Ban Ki-moon called for the perpetrators of "this appalling violence" to be brought to justice, while a European Union spokesperson sent a message of "continuing solidarity and support to Turkey".
- German, US warnings -
Turks are reeling from the increased frequency of the terror attacks that risk dealing a knockout blow to the country's vital tourism trade.
The radical Kurdistan Freedom Falcons (TAK) claimed responsibility for last weekend's attack on a busy transport hub in Ankara that killed 35 people.
TAK, which also claimed a February suicide bombing targeting troops in Ankara that killed 29 people, has ties to the outlawed Kurdistan Workers' Party (PKK) against which the Turkish army is fighting in a major offensive.
IS was blamed for three of the previous five attacks, including an attack on a peace rally in Ankara in October that claimed 103 lives.
On Thursday, Germany had closed its embassy in Ankara and consulate in Istanbul, citing "very serious" indications of planned attacks.
The US embassy in Ankara had also warned its citizens in Turkey to avoid public gatherings ahead of Kurdish Nevruz (New Year) celebrations on Sunday and Monday, a rallying point in the past for pro-Kurdish protests.
In July, the PKK resumed its three-decade-long insurgency against the Turkish state following the collapse of a shaky two-year ceasefire. The conflict has claimed some 40,000 lives.