Two Americans were among those killed when a suicide attacker detonated a bomb in a popular shopping district of Istanbul, Turkey, on Saturday, the White House said.
Turkey's Health Ministry said four people were killed and 39 were wounded, mostly foreigners. Three of the dead were described as being from Israel and one from Iran, while the nationality of the bomber was not immediately released.
The State Department identified the dead Americans as Yonathan Suher and Avraham Goldman. "We offer our sincere condolences to their family and friends," State Department spokesman John Kirby said in a statement.
It was not immediately clear whether the Americans held a dual citizenship, but the White House said they were among the five reported fatalities.
"The United States condemns in the strongest possible terms today's terrorist attack in Istanbul, Turkey," Ned Price, spokesman for the National Security Council, said in a statement. "Two American citizens were among those killed in this heinous attack. Our thoughts and prayers go out to the families and loved ones of those killed, and we wish a speedy recovery to those injured."
A spokesman for the Israeli Foreign Ministry earlier said 10 Israelis were among the wounded and at least 10 were still missing.
Kurdish militants are suspected of carrying out the bloody violence and the nation's fourth suicide bombing in Turkey this year hit part of Istiklal Street, a long pedestrian zone lined with global brand name shops and foreign consulates, just a few hundred meters from an area where police buses are usually parked.
Preliminary findings indicate that the outlawed Kurdistan Workers Party (PKK) or an affiliate carried out the attack, a senior Turkish official told Reuters.
"The attacker detonated the bomb before reaching the targeted point because they were scared of the police," the official said, adding the bomber had planned to hit a more crowded spot.
Armed police sealed off the shopping street where half a dozen ambulances had gathered. Forensic teams in white suits scoured the area for evidence. Police helicopters buzzed overhead and panicked shoppers fled the area, ducking down narrow sidestreets.
"My local shopkeeper told me someone had blown himself up and I walked toward the end of the street," one neighborhood resident told Reuters.
"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."
Turkish Prime Minister Ahmet Davutoglu denounced the suicide bombing as "inhumane" and said Turkey would continue its struggle against "centers of terrorism."
"No center of terrorism will reach its aim with such monstrous attacks," he said in a written statement. "Our struggle will continue with the same resolution and determination until terrorism ends completely."
Istiklal Street, usually thronged with shoppers on weekends, was quieter than normal before the blast as more people are staying home after a series of deadly bombings.
Health Minister Mehmet Muezzinoglu confirmed that at least 39 people had been wounded and seven of those were in serious condition. Twenty-four of the wounded were foreigners, he said, including six from Israel, two from Ireland, and one each from Germany, Iran and the United Arab Emirates.
Ireland's Minister for Foreign Affairs and Trade said in a statement that "a number of" Irish citizens were injured, and he is "deeply saddened by today's horrific bomb attack."
Israeli Prime Minister Benjamin Netanyahu said in a statement that two planes are being dispatched to Turkey to bring back the dead and wounded Israeli citizens. He added that intelligence does not suggest Israelis were specifically targeted.
"It is not a secret that we are trying to normalize our relationship with Turkey and (are) hoping this will continue," Netanyahu said.
Saturday's blast was the latest in a series of deadly threats.
A suicide car bombing in the capital Ankara killed 37 people this month. A similar bombing in Ankara last month killed 29 people. A Kurdish militant group has claimed responsibility for both of those bombings.
In January, a suicide bomber killed around 10 people, most of them German tourists, in Istanbul's historic heart, an attack the government blamed on Islamic State.
NATO member Turkey faces multiple security threats. As part of a U.S.-led coalition, it is fighting Islamic State in neighboring Syria and Iraq. It is also battling PKK militants in its southeast, where a 2-1/2-year ceasefire collapsed last July, triggering the worst violence since the 1990s.
The U.S. condemned the latest violence in Istanbul in a statement Saturday, saying the nation "stands in solidarity with our NATO Ally Turkey in combating the common threat of terrorism."