The head of the FBI publicly challenged US President Donald Trump, denying the Republican's claim that former president Barack Obama wiretapped his 2016 election campaign.

FBI Director James Comey also confirmed that his agency has launched a criminal investigation into any collusion between Mr Trump's campaign and Russia.

He told a congressional hearing he had seen no evidence to support a claim by Mr Trump that Mr Obama had wiretapped his campaign headquarters in Manhattan's Trump Tower.

The president created a controversy earlier this month when he tweeted without giving evidence that Mr Obama had wiretapped the campaign as the businessman took on Democrat Hillary Clinton in the presidential race.

"With respect to the president's tweets about alleged wiretapping directed at him by the prior administration, I have no information that supports those tweets," Mr Comey told the House of Representatives Intelligence Committee hearing.

The committee is investigating accusations that Russia tried to influence the election mostly by hacking Democratic operatives' emails and releasing embarrassing information. Russia denies the allegations.

Mr Comey confirmed the FBI has been investigating since last July possible Russian government efforts to interfere in the election, including any links between Mr Trump's campaign and Moscow.

He said that while the Russian government wanted to hurt Mrs Clinton's campaign and help Mr Trump's, intelligence agencies made no judgment on whether the efforts influenced the outcome.

Mr Comey gave no details of the classified investigation and said the fact that it exists does not mean charges will be filed.

US intelligence agencies have concluded that Russia tried to help Mr Trump by hacking leading Democrats.

Mr Comey said Moscow had long been opposed to Mr Trump's election rival, former secretary of state Mrs Clinton.

"I think that was a fairly easy judgment for the (intelligence) community," Mr Comey said.

"Putin hated Secretary Clinton so much that the flip side of that coin was he had a clear preference for the person running against the person he hated so much," he said.

Asked about Mr Comey, White House spokesman Sean Spicer read a series of quotes from officials - some from the Obama administration - who have said they have seen no signs of collusion between Mr Trump's campaign and Russia.

In a tweet before the hearing, Mr Trump wrote: "The Democrats made up and pushed the Russian story as an excuse for running a terrible campaign."

Mr Spicer said he was not aware of any White House official being under investigation by the FBI.