A pipe bomb exploded on an overpass near an Istanbul metro station and injured five people on Tuesday, according to the district mayor. Atilla Aydıner, the mayor of the Bayrampaşa district on the European side of the city, told A Haber television the blast happened during rush hour. Ambulances were dispatched to the scene while metro services were halted after the explosion, the Istanbul municipality said. It's unclear what caused the explosion, but earlier reports said it could be linked to an electrical transformer. Turkey has raised security concerns since 103 people were killed in a suicide bombing attack in the capital Ankara in October.