A nail bomb exploded at London's Warren Street underground station on Thursday, Sky news channel reported.

One was injured in the minor explosion, Sky News quoted a source as reporting.

Three tube lines have been closed down for the moment. Emergency services rushed to three London underground stations in London after reports of explosions two weeks after bomb attacks killed 56 people and injured 700 in the British capital.

London fire brigade said smoke was seen coming from one train and there were also reports of a minor explosion on a bus in the Hackney area of east London. Its windows were blown out.

One witness told Sky TV a passenger on a train near Warren Street underground station had told him there had been a small explosion in a passenger's rucksack.

"The rucksack was blown open by the force of a minor explosion," he said.

(Xinhua News Agency July 21, 2005)