Barack Obama has strongly suggested Russia president Vladimir Putin knew about the email hacking that rocked the US presidential race and urged Donald Trump to back a bipartisan investigation.
"Not much happens in Russia without Vladimir Putin," Mr Obama said in his year-ending news conference.
The US president said he had warned Mr Putin there would be serious consequences if he did not "cut it out," though Mr Obama did not specify the extent or timing of any US retaliation for the hacking, which many Democrats believe contributed to Mr Trump's victory over Hillary Clinton.
Mr Obama also expressed bewilderment over Republican lawmakers and voters alike who now say they approve of Mr Putin, declaring that "Ronald Reagan would roll over in his grave".
Mr Trump has dismissed recent talk about hacking and the election as "ridiculous".
Mrs Clinton has even more directly cited Russian interference with the US election, saying: "Vladimir Putin himself directed the covert cyber attacks against our electoral system, against our democracy, apparently because he has a personal beef against me."
Mr Obama did not publicly support that theory on Friday but he criticised the media for what he called an "obsession" with the flood of hacked Democratic emails that were made public during the election's final stretch.
US intelligence assessments that Russia interfered in the election to benefit Mr Trump have heightened the already tense relationship between Washington and Moscow.
Separately, Mr Obama has blamed Russia for standing in the way of international efforts to stop the civil war in Syria, where government forces have beaten back rebels in their stronghold of Aleppo.
Mr Obama said he feels "responsible" for some of the suffering in Syria but he defended his decision to avoid significant military action there. He said that while military options short of invasion were tempting, it was "impossible to do this on the cheap".
He pinned the bulk of the blame on Russia, as well as Iran, for propping up Syrian president Bashar Assad.
"This blood and these atrocities are on their hands," he said as he addressed reporters from the White House briefing room shortly before leaving on his annual Christmas vacation to Hawaii. The news conference lasted about an hour and a half, longer than usual.
The president is ending his eighth year in office with his own popularity on the rise, though Mr Trump's election is expected to unwind many of his policies. He is leaving his successor a stronger economy than he inherited but also the intractable conflict in Syria and troubling issue of whether Russia was meddling in the US election to back Mr Trump.
US intelligence agencies have concluded with "high confidence" that Russia interfered in the election on Mr Trump's behalf. The president-elect has disputed that conclusion, setting up a potential confrontation with lawmakers in both parties.
The president rejected any notion that the dispute over the origin of the hacking was disrupting efforts to smoothly transfer power to Mr Trump. Despite fiercely criticising each other during the election, the pair have spoken multiple times since the campaign ended.
"He has listened," Mr Obama said of Mr Trump. "I can't say he will end up implementing. But the conversations themselves have been cordial."