ISTANBUL (AFP) - A suicide bombing ripped through a famous shopping street in central Istanbul on Saturday, killing four people and injuring dozens less than a week after an attack by Kurdish rebels left 35 dead in Ankara.

The sixth major attack in Turkey since July targeted Istiklal Caddesi, a pedestrian thoroughfare in the European side of the city that is usually thronged with shoppers and strollers but was still relatively quiet when the bomber struck around 11am (5pm Singapore time).

"This is a suicide attack, a terrorist attack," Istanbul governor Vasip Sahin told reporters at the scene, giving a toll of four dead. The perpetrator was also killed, he said.

Local authorities and media said the victims included three Israelis and one Iranian.

The White House later said that two Americans had been killed in the blast. It was not immediately clear if the two Americans had dual citizenship or were additional fatalities.

Health Minister Mehmet Muezzinoglu said 36 people were injured, seven seriously. They included six Israelis along with two people from Ireland and one each from Germany, Iceland, Iran and Dubai, the health ministry said.

CCTV footage circulated by Turkish media showed a man wearing a long coat, reportedly the bomber, approaching a small group of people outside a local government building directly before the blast.

"All possible avenues (of investigation) are open," a Western diplomatic source told AFP, noting that the target of the attack could have been either tourists or a symbol of the state.

Sahin said the official building appeared to be the intended target.

No group had yet claimed responsibility for the attack.

A waiter working in a nearby restaurant told AFP how he had 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," said the waiter who gave just his first name Mustafa.

He described "people lying all about, it was carnage."

A large stretch of the street was evacuated after the attack. Armed police sealed off the area as ambulances ferried the injured to hospital and a police helicopter hovered overhead.

Deputy prime minister Numan Kurtulmus was among the first senior officials to react, telling reporters the government would "not yield in the face of terrorism" and expected "unqualified support from those who call themselves Turkey's friends" - apparently referencing recent criticism in Europe of hardline President Recep Tayyip Erdogan.

Prime Minister Ahmet Davutoglu called an emergency security meeting in Istanbul.

The US embassy in Turkey said on Twitter it was "saddened" and "shocked" by the attack, which French Foreign Minister Jean-Marc Ayrault called "despicable and cowardly" in a statement.

Singapore's Ministry of Foreign Affairs condemned the attack and said it was "saddened by the loss of innocent lives".

"We convey our deepest condolences to the families of the victims and wish the injured a speedy recovery.

"The Singapore Embassy in Ankara has been contacting registered Singaporeans in Istanbul to verify that they are safe. Our Embassy is also in touch with the local Turkish authorities. Thus far, there are no reports of any Singaporeans affected by the incident."

Germany's foreign ministry urged German tourists in Istanbul to exercise extreme caution and remain close to their hotels.

Turkey, which has been rocked by a string of attacks since last summer - some the work of Islamic State in Iraq and Syria (ISIS) militants, others claimed by Kurdish rebels - had been on high alert ahead of Kurdish New Year celebrations on Monday.

One rebel group, the Kurdistan Freedom Falcons (TAK), had claimed responsibility for a March 13 attack on a busy transport hub in Ankara that killed 35 people.

TAK, which also claimed a suicide bombing in Ankara in mid-February that killed 29 people and had staged a suicide attack in the area of Saturday's Istanbul attack back in 2010, has ties to the outlawed Kurdistan Workers' Party (PKK) against which the Turkish army is engaged in a major offensive.

The TAK said last week's Ankara bombing by a 24-year-old female student aimed to avenge members of the Kurdish minority killed during the military's battle against the PKK and had not meant to target civilians.

Five men were remanded in custody in Ankara Friday over their suspected involvement in the attack.

Turkey has also been repeatedly targeted in recent months by IS members operating out of neighbouring Syria.

ISIS was blamed for three of the last six attacks, including a suicide bombing in January in Istanbul in which 12 German tourists were killed and an attack on a peace rally in Ankara in October that claimed 103 lives.

On Thursday, Germany closed its embassy in Ankara, its consulate in Istanbul and German schools in both cities, citing "very serious" indications of planned attacks against its missions.

The US embassy in Ankara had also warned its citizens in Turkey to avoid public gatherings during the Kurdish Nevruz (New Year) holiday.

Embarrassed by accusations of security lapses, Erdogan has called for journalists, activists and intellectuals who criticize the army's tactics in battle-scarred Kurdish towns and cities to be designated "accomplices" of terror, causing an outcry of protest over dwindling free speech.

The Turkish military meanwhile carried out fresh air strikes against PKK hideouts across the border in northern Iraq on Friday and Saturday, the army said.

The PKK resumed its three-decade-long insurgency against the Turkish state last summer following the collapse of a shaky two-year ceasefire.

The conflict has claimed some 40,000 lives in over the decades.

*** Singaporeans in Turkey are advised to exercise vigilance and to follow the instructions of the Turkish authorities. Singaporeans who require consular assistance while in Istanbul are advised to contact the Singapore Embassy in Ankara or the 24-hour MFA Duty Office at:

Embassy of the Republic of Singapore in Ankara

Tel: +90 530 066 7311