In the midst of escalating regional tensions, British Prime Minister Boris Johnson said Wednesday that Russia would be making a “tragic mistake” if it embarked on “military adventurism” on the borders of Poland and Ukraine.

“What we have to do is make sure that everyone understands that the cost of miscalculation on both Ukraine’s and Poland’s borders would be enormous,” Johnson told a panel of senior UK lawmakers.

“I believe the Kremlin would be making a tragic, tragic mistake if it thought there was anything to be gained by military adventurism.”

The British prime minister made the remarks during a wide-ranging question-and-answer session with a watchdog committee of MPs, as the West grows increasingly concerned about Russian troop movements near the Ukrainian border.

Moscow has also been accused of aiding in the orchestration of a crisis that has trapped hundreds of Middle Eastern migrants on the Belarus-Poland border.

Thousands of troops have been deployed on both sides, and Belarus recently conducted joint exercises with Russian paratroopers.