The Kremlin says it wants an apology from Fox News over what it calls "unacceptable" comments made by one of the channel's presenters about Russian President Vladimir Putin in an interview with U.S. counterpart Donald Trump.

Fox News host Bill O'Reilly described Putin as "a killer" in the interview with Trump as he tried to press the U.S. president to explain more fully why he respected his Russian counterpart.

O'Reilly did not say who he thought Putin had killed.

"We consider such words from the Fox TV company to be unacceptable and insulting, and honestly speaking, we would prefer to get an apology from such a respected TV company," Kremlin spokesman Dmitry Peskov told reporters Monday on a conference call.

Fox News and O'Reilly did not immediately respond to a Reuters request for comment, but O'Reilly addressed the request on his show Monday night.

"I'm working on that apology," O'Reilly told viewers. "But it may take a little time. You might want to check in with me around 2023."

Trump's views on Putin are closely scrutinized in the United States, where intelligence agencies have accused Moscow of having sponsored computer hacking to help Trump win office and critics say he is too complimentary about the Russian leader.

When commenting on the allegations against Putin in the same interview, Trump questioned how "innocent" the United States itself was, saying it had made a lot of its own mistakes.

"There are a lot of killers. We've got a lot of killers," he said. "What do you think? Our country's so innocent?

Senate Majority Leader Mitch McConnell was among the Trump critics who said there was no comparison between how Russian and U.S. politicians behaved. (Mark Wilson/Getty Images)

'Probably' authorized murder

That irritated some congressional Republicans who said there was no comparison between how Russian and U.S. politicians behaved.

"Putin's a former KGB agent. He's a thug. He was not elected in a way that most people would consider a credible election," Republican Senate Majority Leader Mitch McConnell told CNN's State of the Union.

"The Russians annexed Crimea, invaded Ukraine and messed around in our elections. And no, I don't think there's any equivalency between the way the Russians conduct themselves and the way the United States does."

Putin, in his 17th year of dominating the Russian political landscape, is accused by some Kremlin critics of ordering the killing of opponents. Putin and the Kremlin have repeatedly rejected those allegations as politically motivated and false.

Trump, who has said he wants to try to mend battered U.S.-Russia ties, was asked a question about some of those allegations by Fox Business before he won the White House.

In January 2016, after a British judge ruled that Putin had "probably" authorized the murder of former KGB agent Alexander Litvinenko in London, Trump said he saw no evidence the Russian president was guilty.

"First of all, he says he didn't do it. Many people say it wasn't him. So who knows who did it?" Trump said.