The widow of the Orlando nightclub shooter has been arrested by the FBI in connection with the attack, a US law enforcement official said.

The official said Noor Salman was taken into custody in the San Francisco area on Monday morning and is facing charges in Florida including obstruction of justice.

A Twitter post from the United States attorney's office in Orlando said Salman will make her initial appearance in Oakland, California, on Tuesday morning.

Salman moved to the San Francisco area after her husband, Omar Mateen, was killed in a shoot-out with SWAT team members during the massacre at the Pulse nightclub in Orlando on June 12.

FBI agents repeatedly questioned Salman in the aftermath of the shooting about whether she had advance knowledge of her husband's plans.

She told The New York Times in an interview published in November that she was unaware that Mateen planned to attack the nightclub.

He was the only shooter, and by the time a three-hour stand-off between Mateen and law enforcement had ended, 49 patrons were killed and another 53 people needed hospital treatment.

Mateen pledged allegiance to the Islamic State group in a 911 call to emergency officials during the stand-off.

He also made a series of Facebook posts and searches before and during the attack.

Last month, Salman filed a petition in a California court to change the name of the son she had with Mateen.