In a rare show of unity, senior Republican and Democratic leaders are backing a probe into Russian interference in the presidential election, variously describing the situation as disturbing, unacceptable and alarming.

That position puts the lawmakers on a collision course with president-elect Donald Trump, who has cast doubt on the conclusion of U.S. intelligence agencies that Russia directed cyberattacks to meddle in the campaign. Reports that Russia aimed to tip the election in his favour are "ridiculous," Mr. Trump said.

The standoff amounts to an unusual conflict between a president-elect and members of his own party, weeks before Mr. Trump is set to take office on Jan. 20.

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On Monday, the two most senior Republicans in Congress, Senate Majority Leader Mitch McConnell and House Speaker Paul Ryan, both said that they support an investigation and defended the work of the intelligence community, which Mr. Trump's transition team had publicly criticized.

Mr. McConnell strongly condemned "any foreign breach of our cybersecurity measures" and declared that he had "the highest confidence in the intelligence community and particularly the Central Intelligence Agency."

He also struck a much tougher tone toward Russia than Mr. Trump. "The Russians are not our friends," Mr. McConnell said, and "do not wish us well."

Mr. Ryan, the House Speaker, said in a statement that foreign intervention in the U.S. electoral process was "unacceptable" and that any such intervention by Russia was "especially problematic."

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Mr. Trump, meanwhile, continued to question the premise of the probe. "Unless you catch 'hackers' in the act, it is very hard to determine who was doing the hacking," he wrote on Twitter on Monday. "Why wasn't this brought up before election?" (The Obama administration announced in October that U.S. intelligence agencies had concluded that Russia was trying to interfere in the Nov. 8 election).

In September, U.S. intelligence agencies told senior lawmakers that they believed Russia had directed a series of cyberattacks on U.S. political organizations, but that the ultimate goal of the hacking was unclear. Now, the CIA believes the attacks were an effort to influence the election in Mr. Trump's favour, according to The Washington Post.

In one part of the operation, individuals connected to the Russian government provided thousands of hacked e-mails from the Democratic National Committee to the website WikiLeaks, the newspaper said, describing the conclusions of U.S. intelligence agencies. The correspondence included thousands of messages to and from John Podesta, the chairman of Hillary Clinton's campaign.

On Sunday, four senators – two Republicans and two Democrats – released a statement calling for a joint probe into the alleged Russian activity. "Recent reports of Russian interference in our election should alarm every American," the senators said. "This cannot become a partisan issue. The stakes are too high for our country."

While both Republicans and Democrats in Congress have called for a probe, there is some disagreement on what form the inquiry should take. Mr. McConnell favours an investigation conducted by existing congressional committees, rather than setting up an independent panel. And Mr. Ryan warned that the conclusions of the intelligence community should not call into question "the clear and decisive outcome of this election."

Former intelligence officers described Russian meddling in the election in dire terms. "It is an attack on our very democracy," said Michael Morell, former acting director of the CIA, in an interview posted Sunday on The Cipher Brief, an online security forum. "This is the political equivalent of 9/11. It is huge."

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Mr. Morell said the goal of any investigation should not be to determine whether Russia's efforts actually affected the vote – something he described as unknowable – but to determine exactly what happened and how to prevent it in the future. Mr. Morell publicly supported Ms. Clinton in November's election.

The controversy over Russian interference places the CIA in a delicate position. Its latest reported assessment – that Russia's goal was to assist Mr. Trump – is political dynamite. It also marks the second time in recent months that a federal agency has drawn criticism for actions that affect the political arena.

Less than two weeks before the election, the U.S. Federal Bureau of Investigation said it was reviewing a new set of e-mails in connection with its probe into Ms. Clinton's use of a private e-mail server while secretary of state. Ten days later, the agency said the review had turned up no new information – but the controversy had already dented Ms. Clinton's lead in the polls.

Mr. Podesta, Ms. Clinton's campaign chairman, released a statement on Monday calling on U.S. President Barack Obama to declassify and release information related to Russia's meddling in the election.

"The administration owes it to the American people to explain what it knows regarding the extent and manner of Russia's interference," Mr. Podesta said.