WASHINGTON - US President Donald Trump has responded to the appointment of former FBI director Robert Mueller to lead an investigation into alleged Russian meddling in the election by protesting his innocence and calling for the probe to conclude "quickly".

The appointment of Mr Mueller dramatically raises the stakes for Mr Trump in the multiple investigations into his campaign's ties to the Russians. It follows a swiftly moving series of developments that have roiled Washington, including Mr Trump's abrupt dismissal of the FBI director James Comey and the disclosure that the president urged Mr Comey to drop the bureau's investigation into his former national security adviser Michael Flynn.

In a short statement issued by the White House on Wednesday (May 17), Mr Trump maintained that his campaign did not collaborate with "any foreign entity".

"As I have stated many times, a thorough investigation will confirm what we already know - there was no collusion between my campaign and any foreign entity," he said in the statement.

"I look forward to this matter concluding quickly. In the meantime, I will never stop fighting for the people and the issues that matter most to the future of our country."

Mr Trump, who said in a speech earlier on Wednesday that no politician in history "has been treated worse or more unfairly" has long bristled at the notion that Russia played any role in his November election victory. Moscow has denied US intelligence agencies' conclusion it meddled in the campaign.

Pressure on the White House intensified after Mr Trump fired Mr Comey, who had been leading a federal probe into the matter, and allegations that the president had asked Mr Comey to end the FBI investigation into ties between Mr Flynn and Russia.

That raised questions about whether the president improperly attempted to interfere with a federal investigation.

ADDITIONAL REPORTING BY REUTERS