A car bomb attack targeting a police bus has killed at least 11 people in central Istanbul, officials say.

The explosives were remotely detonated as the vehicle passed through the busy Vezneciler district at the morning rush hour, reports said.

No group has said it carried out the attack.

Violence in Turkey has escalated recently as a result of tensions with Kurdish separatists and the conflict in neighbouring Syria.

The explosion happened near the city's historic Beyazit Square neighbourhood, a major tourist attraction.

Four civilians and seven police officers were among the dead, Istanbul's governor, Vasip Sahin, said. Some 36 other people were injured, he added.

Pictures showed the wreckage of a bus destroyed and the facade of nearby buildings damaged. Armed police were also seen next to the site.

Reports said gunfire was heard in the area after the blast.

Reports said a parked car packed with explosives was detonated by remote control. Photo: Reuters

The so-called Islamic State (IS) group and Kurdish militants have both carried out bloody attacks in Turkey in recent months.

President Recep Tayyip Erdogan has said terror groups are targeting civilians because they are losing their struggle against Turkish security forces.

Turkey is part of the US-led coalition against IS and allows coalition planes to use its air base at Incirlik for raids on Iraq and Syria.

And a two-year-old ceasefire between Turkey and the Kurdish militant PKK broke down last summer.