WASHINGTON, Dec 23 - The Biden administration on Thursday reiterated its warnings to Russia over a potential invasion of Ukraine, with one senior administration official telling reporters that the US is "ready to act if and when we need to."

"We have been clear that there will be significant consequences" if Russia attacks, the official said. The penalties would be coordinated closely with US allies and partners and would severely damage Russia's economy, the official said. Such a move would also result in NATO capabilities being moved closer to Russia.

More Russian military units have been sent to the border area in recent days, sources familiar with the intelligence tell CNN. The latest US intelligence assessments place more than 50 so-called "Battalion Tactical Groups" deployed on and around the Ukraine border.

US and Ukrainian officials have also seen evidence that Russia has begun diverting commercial air and rail systems to support the military effort, though similar air and rail activity was visible in the spring during Russia's last military buildup, which was ultimately pulled back.

In a statement posted to Twitter on Thursday, UK Foreign Secretary Liz Truss used similar language, emphasizing that any incursion would be "a massive strategic mistake and would be met with strength, including coordinated sanctions with our allies." She condemned Russia's "aggressive and inflammatory rhetoric" against Ukraine and NATO, adding that the UK's support for Ukraine was "unwavering."

The US and Russia both indicated on Thursday that the ball is in the other's court when it comes to deterring an invasion of Ukraine, showing little sign that a diplomatic agreement is imminent as Russian President Vladimir Putin has continued to mass forces near Ukraine's border -- despite a direct warning by President Joe Biden earlier this month to de-escalate.

Speaking just hours after Putin's marathon year-end news conference, in which he said Russia's actions will depend on the US's and NATO's "unconditional ensuring of Russia's security today and in the future," the US official said the Biden administration is prepared to discuss some of Russia's concerns.

But the official said Russia has made some demands "that they know we can never agree to." And in an apparent rebuke of Russian officials' insistence in recent days that it would engage only in bilateral talks with the US, the official said any dialogue "must also take place in full coordination with our allies."

Still, the official said, "the Biden administration is willing to engage with Russia through multiple channels as soon as early January." A date and time for the discussions has not been set yet.

In his annual year-end news conference on Thursday, Putin deflected when asked whether Russia plans to invade Ukraine. "How would the Americans react if, on their frontier with Canada, we deployed our missiles?" he told reporters. "It's a question of security, and you know our red lines."

"They just deceived us blatantly. Five waves of NATO expansion. And there you go -- they're now in Romania and Poland, with weapons systems," Putin added. (cnn/ez)