Five people injured as pipe bomb explodes in Istanbul

At least five people were injured when a pipe bomb exploded on an overpass near an Istanbul metro station, the district mayor said, halting some train operations and heightening security fears in Europe's biggest city.

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.

While Haberturk TV initially reported that one person died in the explosion, Istanbul Governor Vasip Sahin said that only one citizen was slightly injured.

There were a number of conflicting reports, however, with local Bayrampasa district Mayor Atilla Aydiner saying that five people were injured.

Turkish authorities have not yet released any official details about the incident, other than to say that the cause of the explosion was still not known.

Haberturk TV reported that a power transformer may have caused the blast, but the report could not be confirmed.

Although Istanbul's metro is largely underground, it runs above ground in some places, including around Bayrampasa.

Photographs on social media showed what appeared to be dozens of people walking alongside overground train tracks after trains had been halted.