AKIPRESS.COM - A car bomb ripped through a police bus in central Istanbul during the morning rush hour on Tuesday, killing 11 people and wounding 36 near the main tourist district, a major university and the mayor's office.

The car was detonated as police buses passed, Istanbul Governor Vasip Sahin told reporters, in the fourth major bombing in Turkey's biggest city this year.

There was no immediate claim of responsibility but Kurdish militants have staged similar attacks on the security forces before, including one last month in Istanbul. However, four people have been detained in connection with a car bomb attack, Turkey's state-run Anadolu agency reported citing police officials.

Security concerns were already hitting tourism and investor confidence. Wars in neighbouring Syria and Iraq have fostered a home-grown Islamic State network blamed for a series of suicide bombings, while militants from the largely Kurdish southeast have increasingly struck in cities further afield.

President Tayyip Erdogan vowed the NATO member's fight against terrorism would go on, describing the attack on officers whose jobs were to protect others as "unforgivable".

Blast killed seven police officers and four civilians, and left 36 people injured, three of them critically, Istanbul Gov. Vasip Sahin told reporters.