The Istanbul governor's office says 10 people died in the blast. Photo: EPA on BBC News
At least 10 people including both foreigners and Turks have been killed in an explosion in a tourist area of Istanbul, Turkish officials say.
A suicide bomber with links to Syria carried out the attack in the Sultanahmet district, according to President Recep Tayyip Erdogan. The explosion took place near the city's famous Blue Mosque.
Fifteen people were also wounded, the Istanbul governor's office said in a statement on its website.
Condemning the attack, President Erdogan said Turkey was the "top target for all terrorist groups in the region" and his country was "fighting against all of them equally".
Eyewitness Murat Manaz told the BBC: "It was a suicide bomb. I went there and saw it and came back to the hotel. There was chaos. Everybody was running somewhere.
"Policemen did not see this coming. They were distressed but at the same time they were trying to evacuate the area because they said there was a possibility that a second bomb could go off."
The authorities are investigating the type of explosive used, the governor's office said.
Norway's foreign ministry said a Norwegian man was receiving treatment in hospital, while Germany's foreign ministry said it had reports that German citizens were among the casualties.
The ministry has on its website (in German) urged German tourists in the city to avoid large crowds and tourist attractions and warned that further violent clashes and "terrorist attacks" were 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 south-eastern 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.