Russia conducted a revolutionary nuclear-capable drone submarine test that could pose a major threat to U.S. ports and harbors, according to reports Thursday. U.S. intelligence agencies reportedly detected the test of the unmanned underwater vehicle during its launch from a Sarov-class submarine on Nov. 27.

Pentagon officials familiar with reports of the test said that the vehicle was code-named Kanyon, but gave out no details about the location or results of the test. According to reports, the drone development program is called the “Ocean Multipurpose System ‘Status-6.’” 

“We closely monitor Russian underwater military developments, but we will not comment specifically about them,” Pentagon spokesman Capt. Jeff Davis reportedly said.

The news about the new drone submarine's development was first reported in September 2015 by the Washington Free Beacon and was later confirmed by the Russian military two months later. However, Russian officials said the program, which was supposed to be a secret, was mistakenly disclosed.

According to U.S. intelligence agencies, the Kanyon secret underwater drone will be equipped with megaton-class warheads—the largest nuclear weapons in existence. 

Russia’s nuclear weapons development in recent years has raised concerns amid a deterioration in the Moscow-Washington ties in the last two years.

Russia’s annexation of Crimea in Ukraine as well as its deployment of troops to the Eastern European nation two years ago worsened the countries’ ties. Russia has also continued bombing of the Syrian city of Aleppo, and was allegedly involved in hacks to influence the U.S. presidential elections — all straining ties between the two superpowers. 

Former Pentagon official Mark Schneider said the test of the underwater nuclear delivery vehicle poses a new strategic threat.

“The Status-6, a nuclear powered, nuclear armed drone submarine, is the most irresponsible nuclear weapons program that Putin’s Russia has come up with,” Schneider, now with the National Institute for Public Policy, said.  “Status-6 is designed to kill civilians by massive blast and fallout.”