A Washington DC police officer working for the US capital's metro system has been charged with providing support and money to the so-called Islamic State (Isis).

The arrest of Nicholas Young is the first time a law enforcement officer has been accused of having links to Isis. The 36-year-old was accused of purchasing 22 gift cards for mobile messaging apps to aid Isis overseas, NBC news reported.

Young had been under surveillance by the FBI since 2009 when DC Metro officials reported his suspicious behaviour to the US domestic intelligence service. Records show that at intervals the police officer had Nazi sympathies and supported Islamic extremism.

In 2011 he travelled to Libya to join rebels in their independence war against 40-year dictator Muammar Gaddafi.

Joshua Stueve, a spokesman for the US Attorney's office, said Young posed no credible threat to the Metro system and was not planning an attack at home, instead focusing on aiding Isis abroad.

On 20 separate occasions, Young met with an FBI agent who posed as an Isis operative. Young advised the agent on how to evade law enforcement detection. He could face a maximum of 20 years in prison if convicted.