As Donald Trump faced searching questions about US relations with Russia during a 75-minute press conference on Thursday, he raised the prospect of nuclear holocaust should relations between the two countries be allowed to deteriorate.
Asked if, given recent Russian activities, he thought he was being tested by Vladimir Putin, Mr Trump responded that it would be "easier" for him to be tough on Russia, but that he believed it would be better for America, and the world, for him to pursue warm relations.
"We're a very powerful nuclear country and so are they," Mr Trump said. "I've been briefed. And I can tell you one thing about a briefing that we're allowed to say, because anybody that ever read the most basic book can say it, nuclear holocaust would be like no other."
"If we have a good relationship with Russia, believe me, that's a good thing. Not a bad thing."
Mr Trump's comments about nuclear war prompted consternation on social media:
In recent days Russian aircraft have buzzed a US destroyer in the Black Sea, a Russian spy ship has been deployed to the US East Coast, and the country has deployed a prohibited nuclear missile.
Speaking at the press conference, Mr Trump excused Mr Putin's behaviour, saying negative media coverage had probably convinced the Russian president that a possible alliance was off.
"If you were Putin now you would say, okay we're back to the old ways. There's no way Putin can do a deal with us."
Mr Trump has talked before of Russia and the US as nuclear power, saying in December that he was ready for a new nuclear arms race. "We will outmatch them at every pass and outlast them all," he said.
He has also called the moment when he received the nuclear codes as "a very sobering moment". "It's very, very, very scary in a sense."
The President also launched an extraordinary attack on the news media in the wide-ranging press conference, during which he insisted that his team was running "like a fine-tuned machine" and lampooned the mainstream media that he said was peddling "fake news".
“I open the paper and I see stories of chaos, chaos,” he said. “It’s the exact opposite. This administration is running like a fine-tuned machine, despite not being able to get my cabinet approved.”
Questions about links between his campaign staff and Russian officials have dogged Mr Trump throughout his candidacy and now his presidency.
The latest controversy led to the resignation of Michael Flynn as National Security Adviser on the grounds that he misled the vice president about his contacts with a Russian ambassador.
Asked again about reports that his aides were in contact with Russian officials before he became president, he called them a "ruse" and "scam" perpetrated by a hostile news media.
"The leaks are absolutely real. The news is fake," Mr Trump told the news conference, referring to media reports that his presidential campaign team had contacts with Russian intelligence officials.
Some senior Republicans have issued their boldest challenge with a vow to get to the bottom of the matter, while Democrats have demanded an independent probe.
The Republican has frequently defended Mr Putin and praised his leadership skills. In September, he compared Mr Putin favourably to Barack Obama, saying: "The man has very strong control over a country. It's a very different system and I don't happen to like the system, but certainly, in that system, he's been a leader, far more than our president has been a leader."