President Barack Obama has put Russia’s Vladimir Putin on notice that the U.S. could use offensive cyber muscle to retaliate for interference in the U.S. presidential election, his strongest suggestion to date that Putin had been well aware of campaign email hacking.

“Whatever they do to us, we can potentially do to them,” Obama declared Friday.

Caught in the middle of a post-election controversy over Russian hacking, Obama strongly defended his administration’s response, including his refusal before the voting to ascribe motive to the meddling or to discuss now what effect it might have had. U.S. intelligence assessments say it was aimed at least in part on helping Donald Trump defeat Hillary Clinton, and some Democrats say it may well have tipped the results in his favor.

Though Obama avoided criticizing President-elect Trump by name, he called out Republicans who he said fail even now to acknowledge the seriousness of Russia’s involvement in U.S. elections.

Obama expressed bewilderment about GOP lawmakers and voters who now say they approve of Putin, and he said unless that changes the U.S. will be vulnerable to foreign influence.

“Ronald Reagan would roll over in his grave,” Obama said as he closed out the year at a White House news conference. Afterward he left for the family’s annual vacation in Hawaii.

Obama declined to state explicitly that Putin knew about the email hacking that roiled the presidential race, but he left no doubt who he felt was responsible. He said that “not much happens in Russia without Vladimir Putin” and repeated a U.S. intelligence assessment “that this happened at the highest levels of the Russian government.”

Obama said he confronted Putin in September, telling the former KGB chief to “cut it out.” That was one month before the U.S. publicly pointed the finger at Russia. Suggesting his directive to Putin had been effective, Obama said the U.S. “did not see further tampering” after that date.

The president has promised a “proportional” yet unspecified response to the hacking of the Democratic Party and Clinton’s campaign chairman. Emails stolen during the campaign were released in the final weeks by WikiLeaks. On Friday, CIA Director John Brennan said in a message to employees that the FBI agrees with the CIA’s conclusion that Russia’s goal was to help Trump win.

Trump has dismissed the CIA’s assessment and talk about Russian hacking as “ridiculous,” while arguing both Democrats and the CIA are trying to undermine the legitimacy of his victory. He made no mention of the hacking — or of Obama — during the latest stop on his “thank you” tour in Orlando, Florida, Friday night.

Clinton has even more directly cited Russian interference. She said Thursday night, “Vladimir Putin himself directed the covert cyberattacks against our electoral system, against our democracy, apparently because he has a personal beef against me.”

Obama said he’d leave it to political pundits to debate the question of whether the hacking swayed the election outcome. He did, however, chide the media for that he called an “obsession” with the emails that were made public during the election’s final stretch.

Accusations of Russian election interference have heightened the already tense relationship between Washington and Moscow. Separately, 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 Aleppo.

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.”

Still, 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.

Meanwhile, the president rejected any notion that the dispute over hacking was disrupting efforts to smoothly transfer power to Trump. Despite fiercely criticizing each other during the election, Obama and Trump have spoken multiple times since the campaign ended.

“He has listened,” Obama said of Trump. “I can’t say he will end up implementing. But the conversations themselves have been cordial.”

(AP)