Russian Navy Yasen-class sub Kazan arrives at its permanent base on Russia's Arctic coast on June 1, 2021.Lev Fedoseyev\TASS via Getty Images Russia's Yasen-class submarines have long been seen as a tough challenge for the US Navy. They are quiet and able to carry torpedoes as well as Kalibr and Oniks cruise missiles. A Russian shipbuilding official said that work is underway to arm them with Zircon hypersonic missiles. The head of a top Russian shipbuilder revealed Monday that work is underway to arm some of Russia's most troubling guided-missile submarines with Zircon hypersonic weapons.Following the fielding of the weapon aboard a Russian navy frigate, "multi-purpose nuclear submarines of the Yasen-M project will also be equipped with the Zircon missile system," Alexei Rakhmanov, general director of the United Shipbuilding Corporation, told the state-run RIA news agency, confirming certain long-standing expectations.He added that "work in this direction is already underway."Russia's Yasen-class nuclear-powered cruise-missile submarines are quiet, difficult to track, heavily armed, and able to conduct attacks against land- and sea-based targets.