US President Donald Trump announced Friday he is under investigation in connection with a probe into alleged Russian election meddling that is gnawing away at his administration - and gathering pace.

The Republican leader also lashed out at the Justice Department's number two, the man tasked with organising the probe.

The furious early morning barrage of tweets - his second in as many days - came as the special counsel investigating Russia's influence over his election pieced together a high-caliber legal team and readied to begin interviews.

Trump is accused of firing FBI director James Comey over his refusal to steer the Russia investigation away from former national security advisor and Trump ally Mike Flynn.

The US president has denied trying to influence the investigation, but has acknowledged in an interview that Russia was on his mind when he dismissed Comey.

"I am being investigated for firing the FBI Director by the man who told me to fire the FBI Director! Witch Hunt" he tweeted from the White House shortly before a day trip to Miami.

Source: Khaleej Times