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.

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.

Suspicion is likely to fall on the outlawed Kurdistan Workers’ Party (PKK) or one of its offshoots who have claimed other attacks on security targets in Turkey this year, says the BBC’s Mark Lowen, in Istanbul.

The explosion happened near the city’s historic Beyazit Square neighbourhood, a major tourist attraction, and an Istanbul university building.

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

“There was a loud bang. We thought it was lightning, but right at that second the windows of the shop came down. It was extremely scary,” an unidentified eyewitness told Reuters news agency.

‘Cold-hearted’

Turkey’s Foreign Minister, Mevlut Cavusoglu, said the attackers were “cold-heartedly” exploding bombs on the holy Muslim month of Ramadan, which is in its second day.

In an interview with state-run TRT television, he also said Turkey was fighting on all fronts against “any form of terrorism.”

The country has been hit by bloody attacks in recent months by the so-called Islamic State (IS) group and Kurdish militants.

President Recep Tayyip Erdogan has said militants 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.

The country has also been waging an offensive against the PKK, which fights for autonomy in the majority-Kurdish areas in Turkey’s south-east.

A two-year-old ceasefire between Turkey and the PKK broke down last summer. Since then, violence has killed hundreds of Turkish security forces, Kurdish fighters and civilians.

Turkey, the EU and US refer to the PKK as a terrorist organisation.

An offshoot of the group, the TAK, carried out two attacks on police vehicles in Ankara this year.

The government and western intelligence groups believe TAK is an alias of the PKK.