Donald Trump's presidential transition team has challenged the veracity of US intelligence assessments that Russia was trying to tip the November election to the Republican.
The CIA has now concluded with "high confidence" that Moscow was not only interfering with the election, but that its actions were intended to help Mr Trump, according to a senior US official.
The assessment is based in part on evidence that Russian actors had hacked Republicans as well as Democrats but were only releasing information harmful to Mr Trump's rival, Hillary Clinton.
Mr Trump's public dismissal of the CIA assessment raises questions about how he will treat information from intelligence agencies as president. His view also puts Republicans in the uncomfortable position of choosing between the incoming president and the intelligence community.
In a statement late on Friday, Mr Trump's transition team said the finger-pointing at Russia was coming from "the same people that said Saddam Hussein had weapons of mass destruction".
On Saturday, spokesman Sean Spicer told CNN there were "people within these agencies who are upset with the outcome of the election".
Mr Spicer denied a New York Times report that Russia had broken into the Republican National Committee's computer networks. The US official who disclosed the CIA assessment to The Associated Press said only that Republican entities had been targeted during the election.
Democratic Senator Chuck Schumer said he would press for a congressional investigation in the new year.
"That any country could be meddling in our elections should shake both political parties to their core," he said. "It's imperative that our intelligence community turns over any relevant information so that Congress can conduct a full investigation."
There was no immediate official response from Moscow. But Oleg Morozov, a member of the foreign relations committee in the upper house of the Russian parliament, dismissed the claim of Russian interference as "silliness and paranoia," according to the RIA Novosti news agency.
Mr Morozov described the allegations as an attempt to force the next administration to stick to Barack Obama's anti-Russian course.
President Obama has ordered a full-scale review of campaign-season cyberattacks to be completed before he leaves office in January.
The investigation will be a "deep dive" into a possible pattern of increased "malicious cyber activity" timed to the campaign season, White House spokesman Eric Schultz said, including the email hacks that rattled the presidential campaign.
It will look at the tactics, targets, key actors and the US government's response to the recent email hacks, as well as incidents reported in past elections, he said.
The president ordered the report earlier in the week and asked that it be completed before he leaves office next month, Mr Schultz said.
"The president wanted this done under his watch because he takes it very seriously," he said. "We are committed to ensuring the integrity of our elections."
The Kremlin has rejected the hacking accusations.
Mr Schultz said the president sought the probe as a way of improving the US defence against cyber attacks and was not intending to question the legitimacy of Mr Trump's victory.
"This is not an effort to challenge the outcome of the election," Mr Schultz said.