Donald Trump has responded with a direct - if brief - "No. No" to a question about whether he had asked the FBI to drop an investigation into his former national security adviser.

The US President also said there was no collusion between himself and Russia.

Mr Trump has been under fire since news emerged this week that he had shared secrets with the Russians and allegedly tried to stop former FBI director James Comey from investigating his former adviser Michael Flynn.

Mr Comey was abruptly fired last week, in a move that shocked Washington.

In a joint news conference with President Juan Manuel Santos of Colombia, Mr Trump was asked whether he had pressured the former FBI boss.


"No. No. Next question," he said.

'Fight, fight, fight!' Donald Trump's pep talk

The Justice Department has appointed former FBI director Robert Mueller as special prosecutor to investigate alleged Russian interference in the 2016 US presidential campaign, including potential links between Moscow and Trump campaign associates.

:: What is a special counsel?

Mr Mueller, who is a widely respected figure who enjoys bipartisan support, will have sweeping powers and the authority to prosecute any crimes he might uncover.

But Mr Trump said the issue was dividing an already polarised nation.

"The entire thing has been a witch hunt," the President said.

"There's been no collusion between certainly myself and my campaign, but I can always speak for myself, and the Russians. Zero," he said during the joint news conference at the White House.

"I think it divides the country."

Is it fair to compare Trump to Nixon?

:: Could Donald Trump be impeached?

Questions surrounding alleged ties to Russia have dogged Mr Trump since before his election.

Gen Flynn was forced to resign his post in February after claims he misled the White House about his contact with Russia.

:: Trump hopes to leave scandals behind as he makes first foreign tip

Mr Trump is leaving on Friday for his first foreign trip as US President, which aides hope can have the effect of refocusing a White House in disarray.

He will travel to the Middle East and Europe.