Barack Obama on Thursday said the United States would retaliate against Russian hacking after the White House accused Vladimir Putin of direct involvement in cyber-attacks designed to influence the US election.
The outgoing US president's remarks dramatically upped the stakes in a dispute between the world's leading nuclear powers over interference that may have swayed last month's tight election in which Republican billionaire Donald Trump defeated Democrat Hillary Clinton.
"I think there is no doubt that when any foreign government tries to impact the integrity of our elections that we need to take action," Obama told NPR radio.
"And we will, at a time and place of our own choosing."
Pointing the finger at the Russian president over meddling in the election also puts the White House on a collision course with Trump, who has become increasingly isolated in questioning Russian involvement in hacks of Democratic Party emails that appeared to have slowed the momentum of Clinton's campaign.
Obama is expected to be peppered with questions about the dispute and any subsequent action when he holds a news conference Friday at 2:15pm (7:15pm GMT) before leaving for a vacation in Hawaii.
"I don't think things happen in the Russian government of this consequence without Vladimir Putin knowing about it," one of his top advisers, Ben Rhodes, said earlier Thursday.
"Everything we know about how Russia operates and how Putin controls that government would suggest that, again, when you're talking about a significant cyber intrusion like this, we're talking about the highest levels of government," he told MSNBC television.
"And ultimately, Vladimir Putin is the official responsible for the actions of the Russian government."
'Not a joke'
White House spokesman Josh Earnest echoed his comments, saying the decision by US intelligence agencies in October to blame "Russia's senior-most officials" was not meant to be "particularly subtle."
Trump caused outrage in July by suggesting that Russia find 30,000 missing emails related to Clinton's use of a private server when she was secretary of state. Surrogates dismissed it as a joke, but he repeated his call on Twitter.
"I don't think anybody at the White House thinks it's funny that an adversary of the United States engaged in malicious cyber activity to destabilize our democracy. That's not a joke," Earnest said on Thursday.
"Mr Trump obviously knew that Russia was engaged in malicious cyber activity that was helping him and hurting Secretary Clinton's campaign," he added.
Asked if the White House believes Russia successfully rigged the election, however, he told reporters that there were a "variety of potential explanations, and that's more of a question for analysts of politics than it is for analysts of intelligence."
Trump, who blasted as "ridiculous" a reported CIA assessment that Russia was trying to help him win, has continued to question Russian involvement in hacks of damaging Democratic Party emails.
He used a campaign-style victory rally in Pennsylvania on Thursday to hit out against Earnest, calling him "this foolish guy" and questioning whether he was talking to Obama to boos from the crowd.
"Having the right press secretary is so important because he is so bad the way he delivers the message," said the president-elect, who has yet to appoint one for his own incoming administration.
"If Russia, or some other entity, was hacking, why did the White House wait so long to act? Why did they only complain after Hillary lost?" Trump tweeted earlier on Thursday.
The 70-year-old billionaire now appears increasingly isolated.
The assessment by the Central Intelligence Agency and the Federal Bureau of Investigation that Russia was involved has been accepted throughout government and by top Republican Senators.
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," he said. "They need to pay a price."
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."
A Kremlin spokesman on Thursday rejected the allegations of Putin's involvement as "ridiculous nonsense."
As for Trump's assertion that no one addressed the issue before the election, Obama has repeatedly said both the Democratic National Committee and the Republican National Committee were apprised of the hacking and the Russian threat well before November 8.
In Congress, senior legislators are demanding a broad investigation and declassification of at least some of the secret CIA and FBI reports.