WASHINGTON (AFP) - Two US citizens were among those killed in a suicide blast that tore through a major shopping district in Istanbul on Saturday, the White House said.

Four people died in the bombing early on Saturday, with local authorities and media saying the victims included three Israelis and one Iranian.

Thirty-six other people were wounded.

It was not immediately clear if the two Americans had dual citizenship or were additional fatalities.

"The United States condemns in the strongest possible terms today's terrorist attack in Istanbul, Turkey," National Security Council spokesman Ned Price said in a statement.

"Two American citizens were among those killed in this heinous attack," he added, without identifying the victims.

"Our thoughts and prayers go out to the families and loved ones of those killed, and we wish a speedy recovery to those injured."

The incident marked the sixth major bombing in Turkey since July. It targeted Istiklal Caddesi, a bustling pedestrian street usually thronged with shoppers, tourists and buskers but which was still relatively quiet when the bomber struck early in the day.

No group has yet claimed responsibility for the explosion but pro-government media blamed it on the Islamic State in Iraq and Syria (ISIS) group, which has been accused of several attacks in Turkey in recent months, including a suicide bombing near the Blue Mosque in January, in which 12 German tourists were killed.

"Turkey has once again suffered a horrific terrorist attack, and we remain steadfast in our support for our Nato ally and partner," Price said.

"These repeated acts of terrorism in Turkey must come to an end," he added.

"We are in close touch with Turkish authorities and reaffirm our commitment to work together with Turkey to confront the evil of terrorism."

Price's remarks largely echoed those of State Department spokesman John Kirby who earlier strongly condemned the bombing, calling it a "vicious terrorist attack" and vowing to stand by Turkey.