The California man who allegedly tried to assassinate President Donald Trump at a Washington gala has been indicted on a fourth charge of assaulting a US officer or employee with a deadly weapon.

The new charge comes in the wake of questions over whether a Secret Service officer - who was shot but not seriously wounded in the attack at the White House Correspondents Dinner on 25 April - was hit by crossfire from another officer.

US Attorney Jeanine Pirro signed off on the new indictment unsealed on Tuesday, which supersedes the previous charges.

Cole Tomas Allen, 31, appeared in court in Washington DC last week. He has not yet entered a plea.

According to court documents filed on Tuesday, a grand jury also indicted Allen on charges of attempting to assassinate the US president as well as two firearms offences - transportation of a firearm and ammunition in interstate commerce with intent to commit a felony, and using, carrying, brandishing and discharging a firearm during a crime.