ISTANBUL • An explosion caused by a home-made bomb rocked an Istanbul metro station during evening rush hour, triggering panic and wounding up to half a dozen people.

The local mayor and the state-run Anatolia news agency confirmed a bomb had caused the blast, which came at a time of growing jitters over security in Istanbul and the risk of a militant attack in Turkey's largest city.

The blast, which was heard in several areas of the city, hit an overpass close to the metro station in the Bayrampasa district.

"Five of our citizens were injured when a pipe bomb left on barriers on the overpass exploded," district mayor Atilla Aydiner told A-Haber television. Security sources were also quoted by the Dogan news agency as saying the blast appeared to have been caused by a home-made bomb.

The metro system in Istanbul was brought entirely to a halt after the blast, the municipality said, although normal service was gradually restored.

Large numbers of ambulances were dispatched to the scene, as commuters were helped to safety, reports said.

Other reports had said the cause of the blast could have been linked to an electrical transformer and that six people had been injured.

Video footage on Dogan showed large sparks shooting out like fireworks from the overpass, lighting up the sky near the height of evening rush hour. Dogan also said a truck on the overpass was damaged by the blast, with holes on its windscreen, while other photos showed a bus with blown-out windows.

Anatolia said it was being investigated whether the bomb had been aimed at a police bus that was damaged in the attack.

There was no immediate claim of responsibility for the blast.

"The cause of the explosion is not clear. We are investigating all possibilities," Istanbul governor Vasip Sahin was quoted as saying by Anatolia.

He gave a lower toll of just one person injured. "It was a large explosion, right in a central location and in a busy place but fortunately there was no loss of life."

Turkey is on alert for attacks after 103 people were killed on Oct 10 when two suicide bombers ripped through a crowd of peace activists in the capital Ankara.

Turkey has, in recent weeks, held many suspected members of the Islamic State in Iraq and Syria (ISIS), with officials saying that they were planning attacks in Istanbul.

The Paris attacks on Nov 13, claimed by ISIS, also raised fears over Istanbul's potential vulnerability to a similar assault.

AGENCE FRANCE-PRESSE