A 29-year-old Moroccan man has been arrested in an FBI sting operation near the US Capitol as he was planning to detonate what he thought were live explosives.

Justice Department spokesman Dean Boyd said the suspect was closely monitored by officers and the purported explosives were deactivated, so people were not in danger.

The man had been under investigation for about a year, according to a counter-terror official and a law enforcement official.

He had overstayed his visitor visa for years, the officials said. The FBI provided him with an unusable gun and inert explosives, the counter-terrorism official said.

The man changed his mind about his intended target several times, but ultimately decided on the Capitol, after scouting out the area a couple of times, the law enforcement official said. The man is not believed to be associated with al Qaida.

Sources said he was not arrested on the Capitol grounds, and the FBI has had him under surveillance around the clock for several weeks.