ISTANBUL - At least 10 people have been killed in an explosion in a district of Istanbul popular with tourists, the city authorities say.
Foreign tourists are reportedly among those hurt in the blast, which occurred in the city's historic Sultanahmet district, near the Blue Mosque.
Fifteen people were also wounded in the blast, the Istanbul Governor's Office said in a statement on its website.
Some reports suggest the explosion was caused by a suicide bomber.
Police have sealed off the area and the authorities are investigating the type of explosive used, the governor's office said.
German media reports say the country's foreign ministry is investigating reports that German citizens have been hurt in the blast.
The ministry has on its website (in German) urged German tourists in the city to avoid large crowds and warned that further violent clashes and "terrorist attacks" are expected across Turkey.
The BBC's Mark Lowen in Istanbul says in recent months there have been sporadic attacks by a far-left group in Istanbul, while violence has soared between Turkish forces and PKK Kurdish militants, mainly in southeast Turkey, after a ceasefire broke down.
Turkey has also been hit by bomb attacks blamed by prosecutors on the so-called Islamic State group.
Two suicide blasts in the capital Ankara in October killed more than 100 people and more than 30 people were killed in an attack near the border with Syria in July.