Estonia said on Thursday that Moscow had briefly sent a fighter jet into NATO airspace over the Baltic Sea during an attempt to stop a Russian-bound oil tanker thought to be part of a shadow fleet defying Western sanctions on Moscow.

Russia deployed a fighter jet in NATO airspace this week after Estonia tried to inspect a so-called shadow fleet tanker passing the EU-member’s economic area, Estonia’s Foreign Minister Margus Tsahkna said.

Estonia warned on Thursday that Russia appears willing to defend with military force its shadow fleet of aging tankers dodging international sanctions to keep oil revenue and equipment flowing, even through the territorial waters of European countries.