Russia is testing a nuclear torpedo in the Arctic.
The "doomsday" device has the power to trigger radioactive tsunamis off America's East Coast.
The Pentagon said it was watching developments "very closely."
Russia is planning to deploy a nuclear-powered missile to the Arctic next summer that's designed to detonate off the coastlines of enemy countries, CNN reported.
Satellite images provided this week to CNN by Maxar, a satellite company, indicated that Russia is testing weapons in the region and building significant military infrastructure in the Arctic - which is increasingly free of ice because of climate change.
CNN reported that Russia would deploy the Poseidon 2M39 missile to its Arctic region next summer. The missile has been referred to in reports as a "doomsday" device because of its devastating power.
The device - images of which first surfaced on Russian state television in 2015 - is an underwater nuclear torpedo designed to hit the ocean floor, kicking up a radioactive tsunami that could spread deadly radiation over thousands of miles of land, rendering it uninhabitable.
Russian President Vladimir Putin requested an update on a "key stage" of the tests in February from his defense minister, and more tests are expected later this year, the Times of London reported.
Russia and NATO countries with a presence in the Arctic region have been increasing their activity there in recent years as rising sea temperatures make it more accessible, Insider's Christopher Woody reported.
Russia has the world's longest Arctic coastline and derives about a quarter of its GDP from the region, and the Northern Sea Route is a valuable shipping corridor for Moscow.
The Pentagon on Monday said it was watching reports of Russian military activities and infrastructure build-ups in the Arctic "very closely."
"Without getting into specific intelligence assessments, obviously we're monitoring it very closely," said Pentagon press secretary John F. Kirby at a briefing Monday.
"Obviously we're watching this, and, as I said before, we have national-security interests there that we know ... we need to protect and defend," Kirby said.
"And as I said, nobody's interested in seeing the Arctic become militarized."
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