A US police officer has been charged with providing support to so-called Islamic State (IS), in the first case of its kind.

Nicholas Young, an officer with the Washington DC Metro transit system, was arrested at the agency's headquarters and is due in court on Wednesday, law enforcement officials said.

He faces 20 years in prison if convicted.

Young, 36, is accused of purchasing 22 gift cards for mobile messaging apps to aid the Islamist militant group abroad, according to court records.

An FBI undercover informant was used to catch Young, who had been under surveillance since 2009, when Metro officials reported suspicious behaviour to authorities.

According to a criminal affidavit, Young met with an FBI source on 20 separate occasions posing as an IS fighter, and advising the source about how to evade being detected by law enforcement.

Young had Nazi sympathies and was a longtime supporter of Islamist extremism. He travelled to Libya in 2011 and told the FBI he fought with rebels seeking to oust dictator Muammar Gaddafi.

Court documents also show that Young knew Zachary Chesser, who was convicted of providing support to Al Qaeda's affiliate in Somalia in 2010.

Young also met Amine El Khalifi, the year before he pleaded guilty to plotting a suicide bomb attack on the US Capitol. Khalifi was later sentenced to 30 years in prison.