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 neighbor and demanded the defense alliance withdraw troops and weapons from Eastern Europe and bar Ukraine, a former Soviet state, from ever joining the NATO.

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