ISTANBUL (Reuters) - Five people were injured when a pipe bomb exploded on an overpass near an Istanbul metro station on Tuesday, the district mayor said, halting some train operations and heightening security fears in Europe’s biggest city.

Forensic officers work on the blast scene in Istanbul, Turkey, December 1, 2015. Five people were injured when a pipe bomb exploded on an overpass near an Istanbul metro station on Tuesday, the district mayor said, halting some train operations and heightening security fears in Europe's biggest city. REUTERS/Osman Orsal

Turkey has been on high alert since more than 100 people were killed by two suicide bombers in the capital Ankara in October, three months after a similar attack at a town near the Syrian border in July left 33 dead.

Tuesday’s blast near the Bayrampasa metro station came at the height of the evening rush hour, district Mayor Atilla Aydiner told A Haber television. Bayrampasa is a residential and industrial area on the European side of Istanbul.

Another broadcaster earlier reported that one person had been killed but Aydiner confirmed only five injuries.

Grainy CCTV footage showed a large flash of light on the overpass followed by what appeared to be burning embers showering to the ground as cars drove below.

Photographs on social media showed what appeared to be dozens of people walking alongside overground train tracks after trains had been halted. Although Istanbul’s metro is largely underground, it runs above ground in some places, including around Bayrampasa.

Tuesday’s blast was much smaller than the ones in Ankara and the town of Suruc near the Syrian border, which are believed to have been carried out by Islamic State militants.

Turkey, a NATO member, has carried out air strikes against the Islamist insurgents in neighboring Syria as part of the U.S.-led coalition fighting the jihadist movement.

It also faces security threats from the outlawed Kurdistan Workers Party (PKK), whose militants often attack police and security in Turkey’s mainly Kurdish southeast, and the far-left Revolutionary People’s Liberation Army-Front (DHKP-C).

The DHKP-C, which is considered a terrorist organization by the United States and Ankara, said that one of its members was involved in an attack on the U.S. consulate in August.