Concerned about Europe's energy vulnerability, the head of NATO said the continent needs to diversify its energy supplies on Sunday. Britain warned it was "highly likely" that Russia, a major natural gas supplier, wanted to invade Ukraine.

Russia has massed some 120,000 troops near its neighbor and demanded the western defense alliance pull back troops and weapons from eastern Europe and bar Ukraine, a former Soviet state, from ever joining the Western defense alliance.

U.S. officials said Saturday Russia's military buildup had been expanded to include supplies to treat casualties of any conflict. Across the border in Ukraine, locals trained as army reservists as the government scrambled to prepare.

Moscow denies any plan to invade but said Sunday it would ask NATO to clarify whether it intends to implement key security commitments after earlier saying the alliance's response to its demands did not go far enough.

"If they do not intend to do so, then they should explain why," Russian Foreign Minister Sergei Lavrov said on state television. "This will be a key question in determining our future proposals."

The U.S. has said it is waiting to hear back from Russia. It says NATO will not withdraw from eastern Europe or bar Ukraine, but it is prepared to discuss topics such as arms control and confidence-building measures.

Washington has spent weeks drafting an agreement with European partners on a strong sanctions package if Russia attacks Ukraine. But the issue is divisive, with Germany urging "prudence."

The EU depends on Russia for around a third of its gas supplies and any interruption would exacerbate an existing energy crisis caused by a shortage.

"We are concerned about the energy situation in Europe because it demonstrates the vulnerability of being too dependent on one supplier of natural gas and that's the reason why NATO allies agree that we need to work and focus on diversification of supplies," NATO Secretary-General Jens Stoltenburg said.

Sanctions plans

Britain said Sunday it would expand the scope of its own possible sanctions in legislation this week to deter Russian President Vladimir Putin.

"We think it's highly likely that he is looking to invade Ukraine. That is why we're doing all we can through deterrence and diplomacy, to urge him to desist," Foreign Secretary Liz Truss told BBC television.

Truss, who is due to visit both Ukraine and Russia in the next two weeks, told Sky News the legislation would enable Britain to hit a much wider variety of targets "so there can be nobody who thinks that they will be immune to those sanctions."

Asked if the new powers could include the ability to seize property in London, Truss said: "Nothing is off the table."

The Center for American Progress, a U.S. think tank, has said Britain would face a challenge uprooting wealthy Russians with Kremlin links from London given close ties "between Russian money and the United Kingdom's ruling conservative party, the press, and its real estate and financial industry."

Asked about this, Truss said: "There's a real threat here to freedom and democracy in Europe. And that is more important than short-term economic gains, both for the United Kingdom but also for our European allies."

A White House official said that the Biden administration plans to spare everyday Russians from the brunt of U.S. export controls if Russia invades Ukraine and focus on targeting industrial sectors. Earlier, a top commerce official said that "key people" would face "massive sanctions."

British Prime Minister Boris Johnson is expected to speak to Putin by phone next week. Stepping up diplomatic efforts after facing criticism for not doing enough, he said he had ordered the military to prepare to help strengthen Europe's borders.

Stoltenburg said NATO had no plans to deploy combat troops to non-NATO member Ukraine in the event of a Russian invasion, adding, "We are focusing on providing support."

Johnson said Sunday the picture on Ukraine's border with Russia was "increasingly concerning."

"I continue to urge Russia to engage in negotiations and avoid a reckless and catastrophic invasion," he said on Twitter.