The head of the FBI publicly challenged U.S. President Donald Trump on Monday, denying the Republican's claim that former president Barack Obama wiretapped his 2016 election campaign and confirming his agency had launched a criminal investigation into any collusion between Trump's campaign and Russia.

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

The president created a controversy in early March when he tweeted without giving evidence that 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," 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.

Comey confirmed the FBI has been investigating since last July possible Russian government efforts to interfere in the election, including any links between Trump's campaign and Moscow. He said that while the Russian government wanted to hurt Clinton's campaign and help Trump's, intelligence agencies made no judgment on whether the efforts influenced the outcome.

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

U.S. intelligence agencies have concluded that Russia tried to help Trump by hacking leading Democrats. Comey said Moscow had long been opposed to Trump's election rival, former secretary of state Clinton.

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

Asked about 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 Trump's campaign and Russia.

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

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

RUSSIAN TIES

Trump has frequently urged better relations with Russia, which has been at odds with the United States in recent years over Moscow's role in Ukraine and the Syrian civil war.

Adam Schiff, the top Democrat on the intelligence committee, detailed activities by Trump advisers or associates with ties to Russia, including former election campaign manager Paul Manafort and Michael Flynn, who was forced out as Trump's national security adviser after talking to the Russian ambassador and then misrepresenting the conversation to Vice President Mike Pence.

"Is it possible that all of these events and reports are completely unrelated and nothing more than an entirely unhappy coincidence? Yes, it is possible," Schiff said. "But it is also possible, maybe more than possible, that they are not coincidental, not disconnected and not unrelated."

Trump's March 4 tweet about wiretapping, which was made without supporting evidence, pulled attention away from the claims of Russian interference in the election. He made the claim two days after Attorney General Jeff Sessions, who had met with Russia’s U.S. ambassador last fall, said he would remove himself from any investigation of Russian interference in the election.

Trump and his advisers have contended in recent weeks that his claims of wiretapping were intended to mean surveillance of the Trump campaign in general but the White House has not provided evidence of surveillance of any kind.

Last week, Trump's spokesman cited a media report that Britain's GCHQ spy agency was behind the surveillance, prompting ridicule in Britain.

The head of the U.S. National Security Agency, Admiral Mike Rogers, told Monday's hearing that the allegation had strained relations with London.

"I think it clearly frustrates a key ally of ours," Rogers said.

Comey warned that Russia would attempt to influence the next U.S. presidential election in 2020 and perhaps the congressional elections next year. "They'll be back in 2020. They may be back in 2018," he said.

The hearing was a rare open congressional intelligence committee hearing and it revealed a stark partisan divide in focus. Majority Republicans concentrated their questions on leaks of classified information - a concern that Trump frequently mentions - and media reports on issues such as contacts between former Trump national security adviser Michael Flynn and Russian officials.

Democrats sought to highlight such links, and shoot down Trump’s wiretapping claim.

(By Patricia Zengerle and Warren Strobel; Additional reporting by Doina Chiacu; Writing by Alistair Bell; Editing by Bill Trott)