The White House said two Americans were among five people killed in a suicide bombing Saturday in the main shopping district of Istanbul.
Two Israelis and an Iranian were also killed in the attack, which wounded at least 36 people.
This was the fourth suicide bombing in Turkey this year. Prime Minister Ahmet Davutoglu condemned the blast as "inhumane."
The attack came amid heightened security across the country, including a government ban on Kurdish New Year celebrations in most cities and towns across Turkey.
White House spokesman Ned Price said the U.S. condemned the attack in the strongest possible terms and affirmed the U.S. commitment to work with Turkey to "confront the evil of terrorism."
No one claimed responsibility for the bombing on Istiklal Street, a busy pedestrian avenue lined with stores and foreign consulates.
WATCH: Video report from the scene
In Israel, Prime Minister Benjamin Netanyahu said intelligence officials were trying to determine whether the bombing specifically targeted Israelis.
Iranian Foreign Minister Mohammad Javad Zarif, who was in Istanbul on an official visit, also condemned the attack and offered condolences to the Turkish government and the nation.
France's Foreign Minister Jean-Marc Ayrault strongly condemned the attack as a “heinous and cowardly act,” adding that Paris stood in solidarity with Turkey.
On high alert
Turkey was already on high alert after recent bombings in the capital, Ankara, claimed by Kurdish rebels — one such blast killed 35 people March 13 — and in the lead-up to the Kurdish spring celebrations Monday. Security forces were continuing a crackdown against the rebel Kurdistan Workers' Party, the PKK, in towns and cities in the predominately Kurdish southeast of Turkey.
Istanbul Governor Vahsip Sahin explained the decision to ban the Newroz Kurdish New Year celebrations.
He said the government had received several requests for celebrations but rejected them in consideration of the safety of people who would attend the events and the safety of other residents of Istanbul. He said the government thought it was not an appropriate time to mark Newroz.
A small official ceremony has been sanctioned for Monday in Diyarbakir and Istanbul, home to the largest Kurdish populations in Turkey. But the country’s main pro-Kurdish party, the HDP, has insisted it will defy the Newroz bans.
Police have been arresting dozens of members of the HDP ahead of Newroz, with celebrations traditionally starting on the Sunday before March 21, attracting hundreds of thousands of Kurds.
Collapsed peace process
During a two-year peace process between the government and the PKK that collapsed last year, Newroz became a public holiday as part of pro-Kurdish reforms. Political scientist Cengiz Aktar warned that the Newroz ban could have serious consequences.
"There is a dialogue of death going between the government and the opposition HDP party, the Kurdish party. Kurds are adamant to celebrate Newroz. For Kurds, probably, it is the most important public celebration that they’ve got. It is very fearful. Personally, I hope that nothing dramatic will happen on Sunday," said Aktar.
Tensions are likely to be heightened further with the Turkish deputy prime minister, Numan Kurtulmus, announcing Friday that as early as next week, parliament will move to lift the parliamentary immunity of deputies of the HDP in order for them to be prosecuted on terrorism charges.