Image copyright Getty Images Image caption Mr Trump again calls the Russia investigation a "witch hunt"

President Donald Trump has appeared to acknowledge he is under investigation in the inquiry into alleged Russian meddling in the US election.

In a tweet, the president also seemed to accuse the US deputy attorney general of pursuing a "Witch Hunt".

He said: "I am being investigated for firing the FBI Director by the man who told me to fire the FBI Director!"

Deputy Attorney General Rod Rosenstein wrote a memo the White House used to justify the firing of the ex-FBI chief.

Mr Rosenstein took over the investigation into whether Russia tried to tip the US election in favour of Mr Trump after Attorney General Jeff Sessions recused himself in March.

The deputy attorney general later appointed Special Counsel Robert Mueller to lead the inquiry.

The FBI and Congress are both looking at whether Trump campaign officials colluded with the alleged Kremlin plot. The inquiries have yet to show evidence of collusion.

Mr Trump also tweeted on Friday morning: "After 7 months of investigations & committee hearings about my 'collusion with the Russians,' nobody has been able to show any proof. Sad!"

His tweets come as Mr Trump marks two years since announcing his plan to run for president.

A storm is brewing - BBC News North America Editor Jon Sopel

When it was first announced a month ago that a special counsel had been appointed to investigate the Trump campaign's links to Russia, the White House issued a restrained statement from the president expressing the hope that the inquiry would be conducted swiftly.

But the more a furious president has taken his defence into his own hands, the deeper the waters swirling around him have become.

He contradicted the Justice Department over the reasons for sacking the FBI Director, James Comey, telling a TV interviewer that he was fired because of "this Russia thing".

That in turn led to reports this week that Donald Trump himself was now under investigation by the special counsel, Robert Mueller, for possible obstruction of justice.

Mr Trump's latest tweet on Friday morning represented a none-too-veiled attack on the deputy attorney general Rob Rosenstein, who appointed the special counsel.

The advice from Mr Trump's family, his legal team and his closest confidantes has been to say nothing and let the special counsel do his work.

It's advice that has been spectacularly rejected.

In other developments:

Mr Rosenstein may have to recuse himself from the Russia investigation if he is called on as a potential witness in the matter, ABC News reports

Vice-President Mike Pence has hired an outside lawyer to handle inquiries into the Trump campaign's potential ties to the Kremlin

The Justice Department's special counsel is also investigating President Trump's son-in-law and senior adviser, Jared Kushner, and his business dealings, according to the Washington Post

Image copyright Twitter

Why is Mr Trump being investigated?

Mr Trump was not under investigation before Mr Comey's firing, but reportedly is now facing scrutiny for obstruction of justice over the circumstances surrounding the ex-FBI chief's sacking, according to US media.

Special Counsel Mueller was reportedly planning to interview intelligence officials on whether Mr Trump got rid of the FBI director, James Comey, in May to hamper an inquiry into his sacked national security adviser, Michael Flynn.

Mr Comey, who had been leading one of several Russia inquiries, testified to Congress last week that the president had pressured him to drop an inquiry into links between Mr Flynn and Russia.

Mr Flynn was forced out in February after he misled the vice-president about his conversations with Russia's ambassador before Mr Trump took office.

Media playback is unsupported on your device Media caption Trump says he made the decision to fire Comey - that's not what the White House originally said.

The White House has said the president "has never asked Mr Comey or anyone else to end any investigation, including any investigation involving General Flynn".

Friday's tweet marked the first time Mr Trump has publicly acknowledged he is under investigation.

Mr Comey said he was "sure" Mr Mueller was looking at whether Mr Trump had obstructed justice, but added that to his knowledge, the president had not tried to impede the Russia inquiry.

Mr Trump is unlikely to face criminal prosecution as a sitting president, but obstruction of justice could be grounds for impeachment.

Who was Mr Trump tweeting about?

Image copyright Getty Images Image caption Mr Rosenstein's memo was initially used to justify the firing of FBI chief James Comey

Though the president did not name Mr Rosenstein in his morning tweet, he appeared to refer to a memo the US deputy attorney general wrote.

The memo, which the White House claimed was the impetus for Mr Comey's dismissal, criticised the FBI chief's failure to follow rules and procedures in his handling of an investigation into Hillary Clinton's private email use while Secretary of State.

Mr Rosenstein appointed Mr Mueller following growing demands from Democrats to appoint an independent special prosecutor to examine the Russia allegations.

It is unlikely the president is referring to Mr Mueller in his tweet because Mr Mueller would not have been in a position to fire Mr Comey.