A suicide bomber thought to be from Syria killed at least 10 people, many of them German tourists, in Istanbul’s historic heart on Tuesday, an attack senior security officials said was probably planned by Islamic State.

President Tayyip Erdogan condemned the bombing in Sultanahmet square, near the Blue Mosque and Hagia Sophia, major tourist sites in the center of one of the world’s most visited cities. He said a Syrian was thought responsible and that the blast would only harden Turkey’s resolve in fighting terrorism.

Deputy Prime Minister Numan Kurtulmus said the bomber had been identified from body parts at the scene, and was a Syrian national born in 1988 whose links were being investigated.

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Sources in Prime Minister Ahmet Davutoglu’s office said most of those killed were Germans and he had spoken by phone with Chancellor Angela Merkel to express condolences. Merkel earlier said she was very worried Germans might be among the victims.

Several bodies lay on the ground in the square, also known as the Hippodrome of Constantinople, in the immediate aftermath. It was not densely packed at the time of the explosion, according to a police officer working there, but small groups of tourists had been wandering around.

“This incident has once again shown that as a nation we should act as one heart, one body in the fight against terror. Turkey’s determined and principled stance in the fight against terrorism will continue to the end,” Erdogan told a lunch for Turkish ambassadors, in a speech broadcast live on television.

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Norway’s foreign ministry said one Norwegian man was injured and was being treated in hospital. The Dogan news agency said nine Germans and one Peruvian were also wounded.

Two senior security officials told Reuters there was a high probability that Islamic State militants were responsible.

Turkey, a NATO member and candidate for accession to the European Union, is part of a U.S.-led coalition against Islamic State fighters who have seized territory in neighboring Syria and Iraq, some of it directly abutting Turkey.

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There was no immediate claim of responsibility but Islamist, leftist and Kurdish militants, who are battling Ankara in southeast Turkey, have all carried out attacks in the past.

The Istanbul governor’s office said the authorities were investigating the type of explosive used and who might have been responsible.

“We heard a loud sound and I looked at the sky to see if it was raining because I thought it was thunder but the sky was clear,” said Kuwaiti tourist Farah Zamani, 24, who was shopping at one of the covered bazaars with her father and sister.