British Defense Secretary John Healey told parliament on Wednesday that the Royal Navy tracked a “Russian spy ship” that passed through British waters, warning Russian President Vladimir Putin: “We know what you’re doing.”

Healey said the Yantar vessel was “used to gather intelligence and map the UK’s critical underwater infrastructure”.

“I also wanted President Putin to hear this message: We see you, we know what you are doing, and we will not shy away from strong action to protect this country,” he told lawmakers.

The ship entered British waters on Monday about 45 miles off the country’s coast, and the Royal Navy sent two vessels to monitor it, Healey said.

“It has been detected hovering over the UK’s critical underwater infrastructure,” the defense secretary said.

He added that the ship is now in the North Sea, having “passed through British waters”.

Healey said the Royal Navy submarine was allowed to surface near Yantar “solely as a deterrent” and “to make it clear that we were secretly watching her every move”. BBC News reported.

Healey said it was the second time Yantar had been spotted in British waters recently, after it was also spotted in November.

British warships have escorted Russian navy ships on several occasions in recent months.

Earlier this month, The Royal Navy Revealed that in late December the frigate HMS Somerset accompanied a Russian naval group as it sailed from the North Sea to the English Channel, although the group remained in international waters.

In November British jets took off for tracking a Russian reconnaissance plane flying close to UK airspace, just days after NATO planes were mobilized when Russian planes were spotted over the Baltic Sea and off the coast of Norway.

Last September, Royal Navy warships spent a week “closely monitoring” four Russian vessels in UK waters, while two Royal Air Force jets flew to intercept two Russian aircraft flying near the UK, the navy said in press release.

Also in September, the U.S. Coast Guard said it tracked a a group of Russian warshipsincluding two submarines, as they crossed into US waters off Alaska in an apparent attempt to avoid sea ice.

A month earlier, a US Coast Guard cutter on a routine patrol around the Aleutian Islands in Alaska a Russian ship came by in international waters but within the exclusive economic zone of the US.