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.

Tensions in the Gulf of Finland escalated after a Russian warplane violated NATO airspace while shielding a shadow fleet tanker from Estonian inspection. Estonia warns Russia may now defend sanctioned vessels with military force.

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.