"America and its allies are not going to be intimidated," he said in remarks at the White House. Putin is "not going to scare us."

Biden then addressed the Kremlin leader directly, pointing his finger into the television camera as he warned against any attack on NATO territory.

"America's fully prepared, with our NATO allies, to defend every single inch of NATO territory," he said. "Mr Putin, don't misunderstand what I'm saying: every inch."

Biden was speaking shortly after Putin presided over a ceremony in Moscow to declare that Russia has annexed four more regions of Ukraine, although Western-armed Ukrainian troops continue to battle to restore control there.

Putin and multiple supporters have suggested that having declared the swath of Ukraine to belong to Russia, the Kremlin could now legitimately resort to nuclear weapons to defend what it says is Russian territory.

Biden called out Putin's "reckless and threats" but dismissed Friday's ceremony as a "sham routine that he put on" to show strength, while instead demonstrating that "he's struggling."

Biden referred to a vote in Congress on Friday to approve $12.3 billion more in military aid for Ukraine, saying "we're going to stay the course. We're going to continue to provide military equipment so Ukraine can defend itself and its territory and freedom."

Addressing the mysterious explosion at the major Nord Stream natural gas pipeline from Russia to western Europe, Biden echoed other Western leaders saying it was "a deliberate act of sabotage."

He did not say who the United States believes is behind the attack but he described Russian allegations that Washington was involved as "disinformation and lies."

"We'll work with our allies to get to the bottom (of) exactly, precisely what happened," he told reporters.

"At the appropriate moment, when things calm down, we're going to be sending divers down to find out exactly what happened," Biden said.

"We don't know that yet exactly," he said, adding that the United States is already working with allies to "enhance the protection of this critical infrastructure."