The US and its European allies are discussing potential responses if Russian President Vladimir Putin takes military action against Ukraine, including fresh sanctions on Moscow and further security assistance for Kiev.
Senior US officials have raised the idea of a package of measures in discussions with their European counterparts over the past week, telling them the Joe Biden administration is already working up a menu of options to counter possible moves by Russia, people familiar with the discussions said.
The planning is at an early stage even within the US and would require more discussions before seeking the support of other countries.
But it comes after US officials raised the alarm that Russia may be weighing a potential invasion of Ukraine as tension flares between Moscow and the EU over everything from migrants to energy supplies.
The Kremlin denies any such intention.
Mr Putin has parked large numbers of troops near the Ukraine border previously, including earlier this year when he sent more than 100,000 troops, as well as tanks, warplanes and warships, to Crimea and other areas.
He resisted for weeks as Kiev and western nations called on him to de-escalate before pulling troops back in late April.
The latest friction comes against a broader backdrop. Not only are troops massing again, but record-high gas prices have left Europe vulnerable to Russian largesse on supplies.
French President Emmanuel Macron defended Ukraine’s sovereignty in a phone call with Mr Putin on Monday in which they also addressed the migrant crisis.
But Mr Putin blasted the government in Kiev in the call for its recent use of a drone against Russian-backed separatists and denounced recent US and allied naval exercises in the Black Sea as “provocative”.
US President Joe Biden and European Commission chief Ursula von der Leyen discussed the Ukraine build-up when they met in Washington last week.
“We see an unusual concentration of troops and we know Russia has been willing to use these types of military capabilities before to conduct aggressive actions against Ukraine,” Nato Secretary General Jens Stoltenberg told reporters on Monday.
The Kremlin last week dismissed talk of a potential invasion as “unfounded”, saying troop movements on its own territory were a domestic matter.
People close to the Kremlin have said Mr Putin aims to send a clear message to the West that Moscow will not tolerate any further expansion of Nato military ties with Ukraine, but is not seeking a wider war.
The spring crisis on the border eased after Mr Biden called Mr Putin and offered to hold a summit, which took place in June. But Ukrainian officials say the Kremlin did not pull back all of the forces it massed at that time.
A similar package of possible countermeasures to the one now under discussion was developed by the US in the spring but not enacted, two people familiar with the issue said. The EU assigned its foreign affairs service to craft options this year to respond to any further Russian moves but those efforts stalled, one of those familiar with the matter added.
This time, the US is preparing to share with allies more of the information that has triggered its worries to build support for a joint response, two of the people familiar with the issue said.
After a meeting with their Ukrainian counterpart in Brussels on Monday, the foreign ministers of France and Germany said in a joint statement that “any new attempt to undermine Ukraine’s territorial integrity would have serious consequences".
Ukrainian Foreign Minister Dmytro Kuleba said: “We agreed that certain communication will be conducted with Russia to demotivate it from any aggressive moves.”
The US sent CIA Director Bill Burns to Moscow in early November to convey its concerns about the build-up. He spoke by phone with Mr Putin but there was no apparent sign of change in direction from the Kremlin.