The US Justice Department has appointed a special counsel to the case involving President Joe Biden's son, Hunter Biden.

Attorney General Merrick Garland on Friday announced the appointment of attorney David Weiss to the post. A special counsel is a prosecutor with a high degree of independence.

Hunter Biden has been under investigation for allegedly failing to pay income tax and possessing a firearm unlawfully while using drugs. Weiss had already been leading the probe.

Biden's son had previously reached an agreement with federal prosecutors to plead guilty to tax charges while avoiding prosecution for the firearm charge.

Republicans have been critical of the handling of the case, saying prosecutors were being soft on the president's family.

Some US media say the appointment of the special counsel is a response to the criticism.

A spokesperson for former President Donald Trump released a statement saying that "if this special counsel is truly independent" he will quickly conclude that Hunter Biden "should face the required consequences."