A Syria-linked suicide bomber has detonated a bomb in a historic district of Istanbul popular with tourists, killing at least 10 people, including nine Germans, Turkish officials said.

Turkish Prime Minister Ahmet Davutoglu said on Tuesday that the bomber was linked to the Islamic State of Iraq and the Levant (ISIL) group.

The explosion occurred at about 10:20am local time on Tuesday morning. Al Jazeera has learned that at least two of the 15 injured were in "critical condition".

Sources in Davutoglu's office earlier told Reuters that the prime minister had spoken by phone with Chancellor Angela Merkel to express condolences.

Davutoglu also told Merkel during the same phone call that the details of an ongoing investigation regarding the suicide attack will be shared with German officials.

"Investigations into the cause of the explosion, the type of explosion and perpetrator or perpetrators are under way," the Istanbul governor's office said in a statement quoted by the Dogan news agency.

Turkish President Recep Tayyip Erdogan condemned the attack and said the attacker was from Syria.

Davutoglu has convened an emergency security meeting of key ministers and officials.

Following the explosion, ambulances rushed to the site.

Police cordoned off the area to protect people against the possibility of a second explosion.

Al Jazeera's Emre Rende, reporting from Istanbul, said police were conducting searches outside the cordoned-off area in case a second bomber was involved.

As of 3pm local time (1300 GMT) on Tuesday, train services near the square had resumed, but police were still gathering more evidence at the site of the attack, he said.

Erdem Koroglu, who was working at a nearby office at the time of the explosion, told NTV television that he saw several people lying on the ground following the blast.

"It was difficult to say who was alive or dead," Koroglu said. "Buildings rattled from the force of the explosion."

The square sits next to the most popular tourist sites in the city, including the 6th-century Greek Orthodox church, the centuries-old Sultan Ahmet Mosque, also known as the Blue Mosque, and the Roman-era Basilica Cistern, an ancient underground water depot.

The blast comes just over a year after a female suicide bomber blew herself up at a police station for tourists off the same square, killing one officer and wounding another.

Turkey has become a target for ISIL, with two bombings last year blamed on the armed group, in the town of Suruc near the Syrian border and in the capital Ankara. The latter killed more than 100 people.

Violence has also escalated in the mainly Kurdish southeast since a two-year ceasefire collapsed in July between the state and the Kurdistan Workers' Party (PKK) armed group, which has been fighting for three decades for Kurdish autonomy.

Speaking to Al Jazeera from Istanbul on Tuesday, Berkay Mandiraci, researcher at the International Crisis Group, said the latest attack was another blow to the Turkish population.

"Most probably we have another ISIL-linked attack here, and we have been warning of such attacks," he said.