Democratic Sen. Joe Manchin of West Virginia issued a stark warning about the geopolitical threat of Russia's presence in the Arctic, saying Moscow's military and economic activity in the region is at the highest level since the Cold War.

"It's unbelievable the commitment they've made," Manchin said on "Face the Nation" Sunday. "We've seen more activity of aircraft flying in those spaces. We've seen submarine activity from Russia, more so than we have during the Cold War."

Despite not seeing any evidence of the Kremlin conducting low-yield nuclear tests in the Arctic — as the Trump administration has suggested — during a recent visit to the region with other lawmakers, the West Virginia Democrat said Russia is undoubtedly expanding its military role in its northernmost territory, home to the largest communities inside the Arctic Circle in the world.

Last week, the director of the U.S. Defense Intelligence Agency indicated in a speech that Moscow was likely staging low-yield nuclear testing at sites in Novaya Zemlya, an island chain in the Arctic Ocean. The claim by one of the highest-ranking intelligence officials in the country represented the first time the U.S. formally — and publicly — accused the Russian government of breaching the parameters set forth by the Comprehensive Nuclear Test Ban Treaty.

Manchin said the expansion he witnessed during his trip is a "game changer" which warrants a U.S. response in the form of more investment and involvement in the Arctic.

"We have got to be on top and the United States has got to start getting involved to make sure that we're a leader up there and not a follower," he said, adding later, "We should be alerted and we should start acting."