Jakarta – Indonesian police have stepped up security at vital installations in Medan after two bombings within 24 hours.

The first explosion rocked the parking lot of the Medan city administration's offices on Monday afternoon, shattering windows and damaging two cars. No one was hurt.

That bomb was attached to a bicycle parked between the cars, police said. The second bomb exploded under a gas pipeline belonging to state energy giant Pertamina.

North Sumatra police spokesman Amrin Karim said the bomb was placed under a gas pipeline across a bridge over the Deli river but the explosion only dented the steel pipe and twisted its protective cage.

A third bomb, which failed to explode, was found at the other end of the bridge. Mr Karim said the blast was a "terror" act.

National police are investigating whether the blasts are linked to earlier explosions, including the October 12 Bali bombings which killed 202 people.

Sources quoted by the state Antara news agency said the Medan municipal offices had received threats related to entertainment at the complex.

Police said Tuesday's bombs were similar to the one that exploded on Monday. North Sumatra governor Rizal Nurdin said on Monday the attack on the local government building was "an act of terror aimed at creating restlessness".

Provincial police chief Inspector General Dewa Astika said the bombings were related but the motive was still unclear. "We have increased security at vital installations and asked the public to be vigilant and report any suspicious things to police," Mr Astika was quoted as saying by the Detikcom online news service.

Indonesia has been hit by a series of bombings during the past few years. The Bali nightclub blasts were blamed on regional terror group Jemaah Islamiah. Asked if the Medan attacks were related to previous bombings, national police spokesman Edward Aritonang said: "We are still gathering information. We're analysing it and we haven't come to any conclusion yet."