Washington (AFP) - US President-elect Donald Trump on Thursday again rejected the CIA's conclusion that Russia manipulated the US election, as the hacking scandal expanded into a hefty foreign policy challenge just five weeks before he takes office.

Even as NBC News reported that US intelligence has concluded that Russian President Vladimir Putin had a direct hand in the hacking plot, Trump took to Twitter to dismiss the issue, which has been roiling since before he defeated Democrat Hillary Clinton on November 8.

"If Russia, or some other entity, was hacking, why did the White House waite [sic] so long to act? Why did they only complain after Hillary lost?" he tweeted.

But the 70-year-old billionaire-turned-president-elect appeared increasingly isolated in his stance as the scandal posed a deep challenge to his aim of resetting Washington's strained relations with Moscow.

The conclusions by key intelligence bodies including the Central Intelligence Agency and the Federal Bureau of Investigation have been accepted throughout the government, including among top members of Trump's Republican Party.

On Wednesday, senior Republican Senator Lindsey Graham said he was informed by the FBI in August that his own campaign had been hacked.

"My goal is to put on President Trump's desk crippling sanctions against Russia. They need to pay a price," he said.

Former CIA director Michael Hayden called Trump "the only prominent American that has not yet conceded that the Russians conducted a massive covert influence campaign against the United States."

"On this particular event, what Mr Trump says about it is the same thing that Mr Putin says about it," he told CNN Wednesday.

- Putin involved? -

Concerns over the extent of the scandal mounted after NBC television reported that US intelligence officials now believe that Putin was personally involved in the election interference operation.

NBC said new intelligence from US and foreign sources "shows that Putin personally directed how hacked material from Democrats was leaked and otherwise used."

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In the last months of the election, documents and emails stolen from Democratic Party computers and accounts were steadily leaked out through the WikiLeaks website, embarrassing the Clinton campaign and arguably costing her crucial votes.

According to other reports, the hacking and leaking also took place on several Democratic congressional campaigns, likewise undermining those candidates in favor of their Republican rivals.

The Kremlin rejected the allegations of Putin's involvement.

"Ridiculous nonsense cannot have any basis," his spokesman Dmitry Peskov told journalists Thursday.

Trump's repeated dismissal of the CIA and FBI reports, and comments he made that former CIA officials call "insulting" to the intelligence community, threaten to undermine his relationship with an essential part of the government even before he takes office on January 20.

As for Trump's assertion that no one addressed the issue before his election win, outgoing President Barack Obama has repeatedly said both campaigns were apprised of the hacking and the Russian threat well before November 8.

"We determined and announced in October that it was the consensus of all the intelligence agencies and law enforcement that organizations affiliated with Russian intelligence were responsible for the hacking of the DNC, materials that were being leaked," Obama said Monday on "The Daily Show."

"So that was a month before the election -- this was not a secret."

The government continues to wrestle with how much detail to divulge on the hacking operation. Obama last week ordered a full review to be completed before Trump's inauguration.

In Congress, some senior legislators are demanding a broad investigation and declassification of at least some of the secret CIA and FBI reports.

- Trump's Russia reset threatened -

Trump's hopes of a new rapprochement with Moscow could face trouble if the intelligence holds up.

According to NPR public radio, Congressman Jack Kingston, who is close to Trump, was in Moscow earlier this week talking to US businessmen about the possibility of Trump lifting the crippling US economic sanctions placed on Russia for its seizure of Ukraine's Crimea region.

"Trump can look at sanctions. They've been in place long enough," Kingston told NPR.

Asked Thursday on MSNBC television whether Trump recognizes the threat to US interests from Moscow, his spokeswoman Kellyanne Conway said he is nevertheless open to working with Putin.

"What he has said is if you can work with Russia and other folks around the globe with whom we don't agree on everything or most things, if you can work with them on some important and necessary projects, if you will, or undertakings like defeating radical Islamic terrorism and stopping ISIS, and if Russia can join with the United States to do something like that, we'll listen."

Trump was at Trump Tower on Thursday for another round of cabinet search meetings as he seeks to round out his team. He will head to Pennsylvania in the evening for his latest "thank you" rally.