ISTANBUL -- 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. At least six Germans were among the wounded, a news agency reported.
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.
Omer Celik, the spokesman for Prime Minister Ahmet Davutoglu's ruling party issued a statement condemning what he called "a heinous attack."
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. The Norwegian Foreign Ministry told Norway's news agency NTB that the Norwegian tourist was slightly injured and was being treated in a local hospital.
It was not immediately known if any foreigners were among the dead.
Germany warned its citizens to avoid crowds outside tourist attractions in Istanbul, saying on a government website that further violent clashes and "terrorist attacks" are expected across Turkey. It also urged travelers to stay away from demonstrations and gatherings, particularly in large cities.
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.
Last month, Turkish authorities arrested two suspected Islamic State militants they said were planning suicide bombings during New Year's celebrations in the capital Ankara.
Fraser reported from Ankara, Turkey. Mehmet Guzel in Istanbul contributed.