The widow of the man who killed 49 people last year in a gay nightclub in the US city of Orlando has been formally charged with a terror-related offence and obstruction of justice.

Noor Salman was charged on Tuesday with aiding and abetting Omar Mateen in the months before the June 12 massacre at the Pulse - the deadliest mass shooting in modern US history.

Salman, 30, stood before a federal judge in Oakland, California, under tight security. She did not enter a plea before she was led back to jail.

"She knew he was going to conduct the attack," Roger Handberg, federal prosecutor, told the judge.

Handberg did not disclose any more details and would not comment after the 15-minute hearing.

The charges

In the indictment unsealed on Monday, Salman was accused of aiding and abetting Mateen in providing material support and resources to the Islamic State of Iraq and the Levant (ISIL) group between April and June of last year.

She was also charged with obstruction, and accused of misleading and lying to police and the FBI during their investigation. The charges carry up to life in prison.

The indictment, which was returned in Florida on January 12, gave no additional details on Salman's actions.

During the attack at the Pulse nightclub, Mateen pledged allegiance to ISIL, also known as ISIS, in a 911 call.

He was killed in a shoot-out with SWAT officers. In addition to the 49 victims killed, 53 people were injured.

The couple lived in Fort Pierce, Florida, at the time of the rampage.

The indictment asks that Salman's assets, including $30,500, be forfeited.

She was taken into custody early on Monday at a house she shared with her mother in Rodeo, a community about 40km northeast of San Francisco.

It was unclear if her son was with her.

It was unclear what she was doing in California and her case is expected to be transferred to Florida.

A judge scheduled another hearing on Wednesday to discuss her possible release before her trial, her transfer to Florida to face the charges, and the appointment of a lawyer.

'Unaware of everything'

Outside the Oakland court on Tuesday, Salman's uncle Al Salman said his niece was innocent and did nothing to help Mateen plan the assault.

"She's a very soft and sweet girl," Salman said. "She would not hurt a fly."

He said Salman was physically and mentally abused by Mateen and that she stayed with him for fear of losing custody of their son.

For her part, Salman told The New York Times in an interview published in November last year that she knew her husband had watched ISIL videos but that she was "unaware of everything" regarding his intent to shoot up the club.

Salman also said he had physically abused her.

Charles Swift, director of the Texas-based Constitutional Law Center for Muslims in America, planned to represent Salman at that hearing, according to public defender John Paul Reichmuth, who served as her lawyer during Tuesday's proceedings.

Linda Moreno, a Florida lawyer who also represents Salman, said after the arrest that Salman "had no foreknowledge, nor could she predict what Omar Mateen intended to do that tragic night".