A woman places a candle at the site of Tuesday's suicide bomb attack at Sultanahmet square in Istanbul, Turkey January 13, 2016.[Photo/Agencies]

ISTANBUL - An Islamic State suicide bomber who killed 10 German tourists in the heart of Istanbul's historic district entered Turkey as a refugee from Syria and went undetected as he was not on any watch lists, Prime Minister Ahmet Davutoglu said on Wednesday.

The bomber, who blew himself up among groups of tourists on Tuesday near the Blue Mosque and Hagia Sophia, the top sites in one of the world's most visited cities, had registered with immigration authorities in the city a week ago.

Turkey has kept an open border to refugees from Syria's civil war and is now home to more than 2.2 million, the world's largest refugee population. But its border has also been used by foreign fighters seeking to join Islamic State or return from its ranks to commit atrocities abroad.

"This individual was not somebody under surveillance. He entered Turkey normally, as a refugee, as someone looking for shelter," Davutoglu told a news conference, adding he had been identified from fragments of his skull, face and nails.

"After the attack his connections were unveiled. Among these links, apart from Daesh, we have the suspicion that there could be certain powers using Daesh," he said, using an Arabic name for Islamic State.

Asked if Turkey planned retaliatory air strikes on Islamic State, Davutoglu said Ankara would act at a time and in a manner that it saw fit. He pointed out the Turkish military had hit Islamic State targets abroad after the Suruc and Ankara attacks.

But he said Russia's entry into the Syrian war was a complicating factor. Turkish war planes have not flown in Syrian air space since Turkey shot down a Russian fighter jet in late November, triggering a diplomatic row with Moscow.

"They (the Russian air force) shouldn't obstruct Turkey's fight against Daesh ... Right now unfortunately there is such a barrier," Davutoglu said. "Certain countries are in an obstructive attitude in terms of Turkey's air bombardments. They should either destroy Daesh themselves or allow us to do it."

A man takes a selfie with the Ottoman-era Sultanahmet mosque, known as the Blue mosque, near the site of Tuesday's suicide bomb attack at Sultanahmet square in Istanbul, Turkey January 13, 2016.[Photo/Agencies]

TOUR GUIDE YELLED "RUN"

Asked about a report in the Turkish media that the bomber had registered at an immigration office in Istanbul a week ago, Interior Minister Efkan Ala earlier confirmed that his fingerprints were on record with the authorities.

The Haberturk newspaper published what it said was a CCTV image of the man, named in some local media as Saudi-born Nabil Fadli, at an Istanbul immigration office on Jan. 5. Turkish officials have said he was born in 1988.

Foreign tourists and Turks paid their respects at the site of the attack early on Wednesday. Scarves with the Bayern Munich football club emblem were left along with carnations and roses at the scene, before Turkish police sealed off the area.

German Interior Minister Thomas de Maiziere, visiting Istanbul, said there were no indications Germans had been deliberately targeted and that he saw no reason for people to change travel plans to Turkey. He said Germany stood resolutely by Turkey's side in the fight against terrorism.

"If the terrorists aimed to disturb, destroy or jeopardise cooperation between partners, they achieved the opposite. Germany and Turkey are becoming even closer," he said, adding there was no link to Germany's role in the fight on terrorism.

Davutoglu praised the German group's Turkish guide who, according to the Hurriyet newspaper, yelled "run" after seeing the bomber standing among the tourists and pulling a pin on his explosives, enabling some of them to get away.

Witnesses said the square was not packed at the time of the explosion, but that several groups of tourists were there.

"I didn't finish the tour, you know, the tour I had bought," said Jostein Nielsen, a wounded Norwegian tourist, as he waited on a stretcher at Istanbul airport, his left leg bandaged.

"I still have to go to the Blue Mosque and the old Turkish Bazaar ... We have no hard feelings towards Turkey. We know there are some mad people out there," he said.

DETENTIONS CONTINUE

Davutoglu said the security forces had detained four people suspected of links to the suicide bomber, and that six of those wounded were still in hospital. The German foreign ministry said earlier five Germans were still in intensive care.

A Peruvian national was also injured in the blast.

Turkey has rounded up hundreds of suspected Islamic State members since launching what it called a "synchronised war on terror" last July, raids which continued on Wednesday.

Since the attack, police have detained a total of 65 people including 16 foreign nationals in six Turkish cities, the Dogan news agency reported.

The Russian foreign ministry confirmed three of those detained were Russian nationals, but it was not immediately clear whether there was any connection to the Istanbul attack, for which there has been no claim of responsibility.

Turkey has faced criticism at home and abroad for failing to do more to fight Islamic State networks, but Ala, the interior minister, defended Turkey's record, saying 200 suspects had been detained just a week before the Istanbul blast.

He said Turkey, which has repeatedly called on foreign intelligence agencies to do more to prevent would-be jihadists from travelling to its shores, had detained 3,318 people for suspected links to Islamic State and other radical groups since Syria's conflict began. Of that number, 847 were subsequently arrested, most of them foreigners.