On Thursday, NATO's security general warned of a "dangerous time" for Europe as Russia builds up forces near Ukraine and conducts joint military drills in Belarus, but he reiterated an offer to have talks with Moscow.

Jens Stoltenberg, speaking alongside British Prime Minister Boris Johnson, said he had written to Russian Foreign Minister Sergei Lavrov, offering to discuss greater transparency around drills as well as nuclear arms control.

"This is a dangerous time for European security," Stoltenberg said at a joint news conference with Johnson. "The number of Russian forces is increasing. The warning period for a potential attack is decreasing."

NATO - News: NATO Secretary General welcomes US announcement to strengthen  military presence in Europe, 02-Feb.-2016

"NATO is not a threat to Russia, but we must prepare for the worst while remaining strongly committed to finding a diplomatic solution," Stoltenberg said.

He stated that Russia must choose between a diplomatic solution to the Ukraine conflict or economic sanctions from the West, as well as an increased military deployment of NATO forces in the alliance's eastern countries.

"Russia has a choice: they can choose a diplomatic solution - and we're ready to sit down - or they can choose confrontation and pay a larger price," he said.

"Economic sanctions will be imposed. There will be a greater NATO military presence in the eastern part of the alliance, and the UK will play a significant role in that."