An ISIS suicide bomber who killed 10 German tourists in Istanbul entered Turkey as a refugee and his movements were not monitored since he was not on any watch lists, Turkish 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 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. 

Asked whether Turkey planned air strikes against Islamic State in Iraq and Syria in response to the bombing, Davutoglu said Turkey would act at a time and in a manner that it saw fit.

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A Turkish police officer stands guard as Turkish Prime Minister Ahmet Davutoglu and German Interior Minister Thomas De Maiziere visit people wounded during the Jan. 12 suicide bombing in Istanbul. (Huseyin Aldemir/Reuters)

"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.

Turkey accuses Syrian President Bashar al-Assad and his allies, including Iran and Russia, of co-operating with ISIS in the Syrian regime's effort to destroy opposition forces.

Turkey, which like Germany is a member of the U.S.-led coalition against ISIS, has become a target for the radical Sunni militants.

It was hit by two major bombings last year blamed on the group, in the town of Suruc near the Syrian border and in the capital Ankara, the latter killing more than 100 people in the worst attack of its kind on Turkish soil.

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.

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.

Guide's warning helped some escape 

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.

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.

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