Turkish police block streets after an explosion in Istanbul’s Sultanahmet district, a main tourist area, on Tuesday. (Reuters)

A suicide bomber believed to be linked to the Islamic State struck the historic heart of Istanbul on Tuesday, killing at least 10 people in what would be the group’s first major attack on Turkey’s vital tourism industry.

The bombing, which injured 15 others, took place in the shadow of the city’s famous nine-domed Blue Mosque, which draws visitors from around the world. Most of the victims were German nationals, Turkish officials said.

The targeting of Turkey’s tourism trade puts the group on a more direct collision course with the Turkish state, which has been criticized for not doing enough to prevent militants from using the country as a crucial route for recruits, supplies and oil smuggling.

But the militants appear increasingly desperate to strike overseas as they lose territory in Iraq and Syria.

“This terror organization, the assailants and all of their connections will be found and they will receive the punishments they deserve,” said Turkish Prime Minister Ahmet Davutoglu. He said the attacker was a member of the Islamic State, though the group has not asserted responsibility for the attack.

The blast occurred just before 10:30 a.m. in the Sultanahmet district, an area that includes the 400-year-old Blue Mosque; Hagia Sophia, a former Byzantine-era basilica; and the lavish Ottoman Topkapi Palace.

The attacker, identified by Turkish authorities as a 28-year-old of Syrian origin, mingled with a group of German tourists as they gathered near the Obelisk of Theodosius, an ancient Egyptian monolith brought to Istanbul — then known as Constantinople — in the 4th century.

Turkish news outlets later identified the attacker as Nabil Fadli, adding that he had been born in Saudi Arabia.

Eight of those killed were Germans, according to authorities. In addition, officials said that at least 15 people were injured in the explosion, including nine Germans and other foreigners — among them a Peruvian and a Norwegian.

“Today Istanbul was hit. Paris has been hit. Tunisia has been hit. Ankara has been hit before,” German Chancellor Angela Merkel said in Berlin. “International terrorism is once again showing its cruel and inhuman face today.”

The White House also condemned the “heinous attack,” which it said “struck Turks and foreign tourists alike.” In a statement Tuesday, National Security Council spokesman Ned Price said that the United States stands with NATO ally Turkey, a “valued member” of the U.S.-led coalition against the Islamic State, and pledges “our ongoing cooperation and support in the fight against terrorism” in the face of the Istanbul attack.

Directly and indirectly, tourism makes up about 12 percent of Turkey’s gross domestic product, according to the World Travel and Tourism Council, an international travel-industry organization, with the country welcoming about 40 million tourists a year.

The vast plazas and surrounding streets of Sultanahmet, normally busy with merchants, vendors and visitors, were quiet late Tuesday, with many restaurants empty. The area immediately around the blast was cordoned off by police.

Yahya Ibrahim, an imam from Perth, Australia, was visiting with his wife and three children when the blast hit.

“We were just about to head into the mosque but then decided to have breakfast first,” he said. A “huge boom” reverberated through the streets, he said, which his 9-year-old daughter thought was thunder.

Ibrahim said that the attack wouldn’t deter them from visiting again. “On a theological basis, it’s a perversion of Islam,” he said of the Islamic State’s ideology. “On a practical level, it’s murder.”

The Islamic State probably has the capability to launch an extended terror campaign against Turkey, said Firas Abi Ali, a senior analyst at IHS, a global risk analysis firm.

“Its territorial losses in Iraq and Syria may well have led the group to assess its needs to expand its influence and capability in Turkey,” he said. “If today’s attack was perpetrated by the Islamic State, it would reflect a shift in the group’s strategy and herald a broader campaign against Turkey.”

However, such a move “will likely provoke a significant backlash by the Turkish government,” he said.

Turkish forces have not directly intervened in the Syrian conflict, but Ankara has been under pressure from Western governments to crack down on the cross-border flow of people and supplies to Islamic State strongholds in Syria. Last summer, Turkey opened its Incirlik air base to U.S. warplanes carrying out airstrikes against Islamic State positions in Syria.

Turkey is also a key backer of rebel groups opposing Syrian President Bashar al-Assad, and it has recently stepped up its ­decades-old fight against Kurdish separatists.

Turkish police conducted raids in five provinces on Jan. 12, and detained suspected militants, although it is unclear whether the raids are linked to the deadly suicide bomb attack in Istanbul earlier in the day. (Reuters)

In the past, Turkish groups have staged their own attacks in the country. A year ago, a Chechen woman believed to be linked to militant factions blew herself up outside a police post in Sultan­ahmet, killing one police officer.

There were also two major suicide attacks on peace activists in the country’s southeast last year, killing more than 100 people. The government blamed the Islamic State for those explosions, but the militant group never asserted responsibility.

Cunningham reported from Baghdad. Brian Murphy and William Branigin in Washington contributed to this report.