A US federal inquiry into possible links between Russia and the Trump campaign will be led by former FBI director Robert Mueller.

The appointment comes amid a growing Democratic outcry for someone outside the US Justice Department to handle the politically charged investigation.

It follows the claims that fired FBI chief James Comey wrote in a memo that President Donald Trump had asked him to end an investigation into former national security adviser Michael Flynn.

According to the reports, Mr Trump told Mr Comey: "I hope you can let this go."

In a statement, the White House moved to deny the reports, saying: "The president has never asked Mr Comey or anyone else to end any investigation, including any investigation involving General Flynn.

"This is not a truthful or accurate portrayal of the conversation between the president and Mr Comey.”

Michael Flynn resigned from his position in February, amid claims he misled the administration about his contacts with Russia. 

The FBI, meanwhile, has been carrying out an investigation into alleged links between the Trump campaign and Russia.

Earlier Mr Trump claimed that "no politician in history" has been "treated worse or more unfairly" than he has.

He was speaking on the day he faced calls to be impeached over his handling of the Comey scandal and his relationship with Russia.

Offering "advice" to new recruits at a coastguard graduation ceremony in New London, Connecticut, he quickly turned the focus on his recent difficulties.

"Over the course of your life you will find that things are not always fair," he said.

"You will find that things happen to you that you do not deserve, and that are not always warranted, but you have to put your head down and fight, fight, fight!"

Donald Trump | File photo

Mr Trump added: "Never, ever, ever give up - things will work out just fine.

"Look at the way I've been treated lately - especially by the media.No politician in history … has been treated worse or more unfairly.

"You can't let them get you down. You can't let the critics and the naysayers get in the way of your dreams.
"I guess that's why we won.

"Adversity makes you stronger - don't give in, don't back down and never stop doing what you know is right.
"Nothing worth doing ever, ever, ever came easy."

Earlier, Vladimir Putin said he was willing to hand over a transcript of a discussion between Mr Trump and senior Russian officials.

The US president allegedly disclosed highly classified information about "terrorism and airline flight safety" while meeting foreign minister Sergei Lavrov and ambassador Sergei Kislyak at the White House last week.

However, Mr Putin said Mr Trump did not pass over secrets and dismissed the scandal as "political schizophrenia".