The sound of an explosion was heard on the European side of

The cause and the location of the blast were not immediately clear but CNN Turk television reported that it took place close to the stadium of Istanbul's Besiktas football club.

"I heard two explosions, two sounds and now sirens of ambulances rushing to the scene," said an AFP correspondent in the city.

Turkey has experienced a bloody year of militant attacks in its two biggest cities that have left dozens dead and put the country on high alert.

Kurdish militants have twice struck in Ankara, while suspected Islamic State group suicide bombers have hit Istanbul on three occasions.

In June, 47 people were killed in a triple suicide bombing and gun attack at Istanbul's Ataturk airport, with authorities pointing the finger at IS.

Another 57 people, 34 of them children, were killed in August in a suicide attack by an IS-linked bomber at a Kurdish wedding in the southeastern city of Gaziantep.

The country is also still reeling from a failed July 15 coup blamed on the US-based Islamic preacher Fethullah Gulen that has been followed by a relentless purge of his alleged supporters from state institutions.