Europe needs to diversify its energy supplies, the head of Nato said on Sunday, as Britain warned it was “highly likely” that Russia, the continent’s biggest natural gas supplier, was looking to invade Ukraine.

Russia has massed some 120,000 troops near its neighbour and demanded the western defence alliance pull back troops and weapons from eastern Europe and bar Ukraine from ever joining the western defence alliance.

US officials said on Saturday Russia’s military build-up 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 on 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 US, which has threatened Russia with major new sanctions if it invades Ukraine, has said it is waiting to hear from Moscow. It says Nato will not withdraw from eastern Europe or bar Ukraine but it is ready to discuss arms control and confidence-building measures.

Britain said on 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.