NATO Secretary General Jens Stoltenberg has warned of a dangerous moment for Europe as Russia builds up troops near Ukraine and conducts joint military exercises in Belarus, Reuters reports.

Speaking alongside British Prime Minister Boris Johnson, Stoltenberg said he had sent a letter to Russian Foreign Minister Sergei Lavrov suggesting that more transparency be discussed on the exercise as well as nuclear arms control.

This is a dangerous moment for European security, Stoltenberg said at a press conference with Johnson.

According to him, the number of Russian forces is increasing. The warning time for a possible attack is being reduced.

NATO does not pose a threat to Russia, but we must prepare for the worst while remaining firmly committed to finding a political solution, Stoltenberg said.

According to him, Russia must choose between a diplomatic solution to the crisis over Ukraine or face economic sanctions from the West and increased military presence of NATO forces in the eastern countries of the alliance.

Russia has a choice: either a diplomatic solution – and we are ready to sit down at the negotiating table – but if they choose confrontation, they will pay a higher price, he said.

There will be economic sanctions. NATO's military presence in the eastern part of the alliance will be increased, and the UK is indeed playing an important role in this, the NATO chief added.