U.S. president-elect Donald Trump on Friday praised the "delay" by Russian President Vladimir Putin, apparently referring to Putin's refusal to retaliate for the U.S. expulsion of 35 Russians over their alleged involvement in hacking political groups before the Nov. 8 presidential election.

"Great move on delay (by V. Putin) - I always knew he was very smart!," Trump tweeted Friday.

Great move on delay (by V. Putin) - I always knew he was very smart! —@realDonaldTrump

Putin has condemned a new round of U.S. sanctions against Russia, but said Moscow won't retaliate by expelling American diplomats.

The decision came as a surprise; tit-for-tat expulsions are common diplomatic practice and Foreign Minister Sergey Lavrov had suggested hours before Putin's announcement that Russia would oust 35 American diplomats.

U.S. President Barack Obama on Thursday ordered 35 Russian diplomats to leave within 72 hours, as well as the closure of two facilities, to retaliate against alleged Russian interference in the U.S. presidential election after American political sites and email accounts were hacked. Moscow denies the allegations.

Putin, in a statement on the Kremlin's website on Friday, referred to the new sanctions as a "provocation aimed to further undermine Russian-American relations."

But he said Russia would not expel American diplomats in retaliation, as the Russian foreign minister had suggested Friday.

"The Russian diplomats returning home will spend the new year holidays with their relatives and dear ones," Putin said. "At home. We will not create problems for U.S. diplomats. We will not expel anybody."

Andrey Kortunov heads think tank with close ties to Russia's foreign ministry 5:03

Festering diplomatic showdown

The diplomatic showdown between Washington and Moscow had been festering even before the Nov. 8 presidential election elevated Trump to the presidency, and puts pressure on the billionaire American businessman not to let Russia off the hook after he takes office on Jan. 20.

Putin said he would wait for the actions of Trump, who will officially become commander-in-chief on Jan. 20, before deciding on any further steps in relations with the United States.

Should Trump seek to heal the rift with Russia, he might encounter opposition in Congress, including from fellow Republicans.

Republican John McCain, chairman of the Senate armed services committee, said on Friday that Russia must face a penalty for the cyberattacks, and that is is possible to impose many sanctions.

"When you attack a country, it's an act of war," McCain said in an interview with the Ukrainian TV channel 1+1 while on a visit to Kiev.

"And so we have to make sure that there is a price to pay, so that we can perhaps persuade the Russians to stop these kind of attacks on our very fundamentals of democracy."

A Kremlin Christmas

Russia's government had threatened retaliation, and it continues to deny U.S. accusations that it hacked and stole emails to try to help Trump win. Trump said the U.S. should move on, but in a sign he is no longer totally brushing off the allegations, he also said he planned to meet with U.S. intelligence leaders next week to learn more.

I am inviting all children of U.S. diplomats accredited in Russia to the New Year and Christmas parties in the Kremlin. - Vladimir Putin , Russia's president

Lavrov said early Friday that Russia's Foreign Ministry and other agencies suggested that Putin order expulsion of 31 employees of the U.S. Embassy in Moscow and four diplomats from the U.S. Consulate in St. Petersburg. Another suggestion was to bar American diplomats from using their summer retreat on the outskirts of Moscow and a warehouse south of Moscow.

Tourists take pictures of the Kremlin as they visit the Red Square in downtown Moscow on Friday, Dec. 30. Putin invited U.S. diplomats and their families to the Christmas and New Year's parties at the Kremlin. (Alexander Zemlianichenko/Associated Press)

But in the website remarks, Putin said Russia would not prevent the diplomats' families and children from using the customary rest and leisure facilities and sites during the New Year holidays.

"Moreover, I am inviting all children of U.S. diplomats accredited in Russia to the new year and Christmas parties in the Kremlin," he said.​

Russians celebrate both Christmas and New Year's Eve, but the day before the new year has been the main holiday there since Soviet times.

The sanctions are the strongest actions the Obama administration has taken to date to retaliate for an alleged cyberattack, and more comprehensive than last year's sanctions on North Korea after it hacked Sony Pictures Entertainment.

The new penalties add to existing U.S. sanctions over Russia's actions in Ukraine that have impaired Russia's economy, but had limited impact on Putin's behaviour.

Obama said the response to Russia wasn't over.

"All Americans should be alarmed by Russia's actions," said Obama, who was vacationing in Hawaii. He added, "Such activities have consequences."

He said the U.S. could take further, covert action — a thinly veiled reference to a counterstrike in cyberspace the U.S. has been considering.