A Pakistani-born man was arrested today and charged with trying to help people posing as al Qaida operatives in a plot to bomb subway stations in Washington.

The bombing plot was a ruse over the past six months, the FBI said, but claimed that Farooque Ahmed, 34, handed over video of northern Virginia subway stations and suggested using rolling suitcases rather than backpacks to kill as many people as possible.

The man is also alleged to have offered to donate money to al Qaida's cause overseas.

The FBI said the public never was in danger because agents were aware of Ahmed's alleged activities and monitored him throughout, while the people he thought were al Qaida operatives were actually individuals who "worked on behalf of the government in this matter".

The charges were made public today after Ahmed was indicted under seal by a federal grand jury on Tuesday.

He is accused of attempting to provide material support to a designated terrorist organisation, collecting information to assist in planning a terrorist attack on a transit facility, and attempting to provide material support to carry out multiple bombings to cause mass casualties.

Ahmed, a naturalised citizen, lives in Ashburn, Virginia, outside Washington.

During a brief court appearance in federal court, Ahmed did not enter a plea and was ordered held without bond. He could face up to 50 years in prison if convicted.