ATTENTION EDITORS - VISUAL COVERAGE OF SCENES OF INJURY OR DEATH An injured policeman lies on a stretcher at the site of an explosion in central Istanbul, Turkey, December 10, 2016. REUTERS/Murad Sezer - RTX2UG03

Wounded police officers are helped after a blast in Istanbul, Turkey, December 10, 2016. REUTERS/Murad Sezer - RTX2UGEW

Relatives mourn next to a coffin of a Turkish police officer killed in Saturday's blasts in Istanbul, Turkey, December 11, 2016. REUTERS/Murad Sezer - RTX2UHVH

Police arrive at the site of an explosion in central Istanbul, Turkey, December 10, 2016. REUTERS/Murad Sezer - RTX2UFWD

A man looks on from a broken window in front of the scene after a blast in Istanbul, Turkey, December 11, 2016. REUTERS/Osman Orsal - RTX2UHH3

A member of Turkish police special forces stands guard next to the Eyup Sultan mosque in Istanbul, Turkey, December 11, 2016. REUTERS/Murad Sezer - RTX2UIDV

Police arrive at the site of an explosion in central Istanbul, Turkey, December 10, 2016. REUTERS/Murad Sezer - RTX2UFYJ

Police extinguish a burning car using a water cannon after a blast in Istanbul, Turkey, December 10, 2016. REUTERS/Murad Sezer - RTX2UGDR

A member of the police special forces stands guard near the scene after a blast in Istanbul, Turkey, early December 11, 2016. REUTERS/Murad Sezer - RTX2UGBL

Turkish police officers carry a coffin of a fellow officer during a ceremony for police officers killed in Saturday's blasts in Istanbul, Turkey, December 11, 2016. REUTERS/Murad Sezer - RTX2UHR6

Workers place Turkish flags in front of Vodafone Arena near the site where police officers were killed in Saturday's blasts in Istanbul, Turkey, December 11, 2016. REUTERS/Murad Sezer - RTX2UHVG

Police and ambulances arrive at the site of an explosion in central Istanbul, Turkey, December 10, 2016. REUTERS/Murad Sezer - RTX2UFYL

A woman kisses a police car at the scene of Saturday's blasts in Istanbul, Turkey, December 11, 2016. REUTERS/Osman Orsal - RTX2UIGI

Police arrive at the site of an explosion in central Istanbul, Turkey, December 10, 2016. REUTERS/Murad Sezer - RTX2UFX5

A damaged vehicle is seen after a blast in Istanbul, Turkey, December 10, 2016. REUTERS/Murad Sezer - RTX2UGLB

Turkish policemen at the site of an explosion in central Istanbul, Turkey, December 10, 2016. REUTERS/Murad Sezer - RTX2UFWV

An ambulance is parked near the scene after a blast in Istanbul, Turkey, early December 11, 2016. REUTERS/Murad Sezer - RTX2UGAV

People carry the flag-draped coffins of police officers killed in Saturday's blasts in Istanbul, Turkey, December 11, 2016. REUTERS/Murad Sezer - RTX2UHNY

Asiye Capat (R), mother, and Yurdagul Capat (L), sister of Yakup Capat, a Turkish police officer killed in Saturday's blasts in Istanbul, mourn during his funeral ceremony in Ankara, Turkey, December 12, 2016. REUTERS/Umit Bektas - RTX2UNEY