An explosion killed at least 10 people and injured 15 others Tuesday morning in a historic district of Istanbul popular with tourists, the Istanbul governor's office said.
The cause of the explosion in the Sultanahmet district was being investigated, but state-run TRT television reported it was likely caused by a suicide bomber. A government official, speaking on condition of anonymity because of government rules, said the explosion was believed to be "terror-linked." He did not provide further details.
The explosion, which could be heard from several neighborhoods, was at a park that is home to a landmark obelisk, some 25 meters (yards) from the historic Blue Mosque.
Turkey's Dogan news agency reported that at least six Germans, one Norwegian and one Peruvian were among the wounded, and Seoul's Foreign Ministry told reporters via text message that one South Korean had a finger injury.
Police sealed the area, barring people from approaching in case of a second explosion, and a police helicopter hovered overhead.
The Sultanahmet neighborhood is Istanbul's main sightseeing area and includes the Topkapi Palace and the Haghia Sophia museum.
Erdem Koroglu, who was working at a nearby office, told NTV television he saw several people 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."
Turkish Prime Minister Ahmet Davutoglu immediately convened a security meeting with the country's interior minister and other officials.
As with previous attacks, authorities imposed a news blackout, barring media from showing images of the dead or injured or reporting any details of the investigation.
Turkey suffered two major bombing attacks last year, both blamed on the Islamic State group.
More than 30 people were killed in a suicide attack in the town of Suruc, near Turkey's border with Syria, in July.
Two suicide bombs exploded in October outside Ankara's main train station as people gathered for a peace rally, killing more than 100 in Turkey's deadliest-ever attack. The prosecutor's office said that attack was carried out by a local Islamic State cell.
Fraser reported from Ankara, Turkey. Mehmet Guzel in Istanbul contributed.