U.S. Northern Command has said American forces « stand ready » to deal with any threats after a Russian submarine surfaced off the coast of Alaska on Thursday. The incident reportedly occurred during Russian war games in the region.

The Associated Press reported Thursday that the unnamed Russian vessel surfaced for an unknown reason in international waters close to Alaska. It follows multiple reports by commercial shipping in the region regarding Russian naval activity.

USNORTHCOM and the North American Aerospace Defense Command are monitoring the submarine and other Russian activity, spokesperson Bill Lewis told AP from the command’s headquarters at Peterson Air Force Base in Colorado.

The USNORTHCOM Twitter channel said it was « closely monitoring the Russian submarine that surfaced near Alaska today. » It added: « We closely track vessels of interest, including foreign military naval vessels, in our area of responsibility. »

USNORTHCOM said the Russian maritime activity « is taking place in international waters well outside the U.S. territorial sea. »

Multiple commercial ships raised concerns with the U.S. Coast Guard on Wednesday regarding the Russian naval vessels in the area. Coast Guard spokesperson Kip Wadlow said Thursday: « We were notified by multiple fishing vessels that were operating out the Bering Sea that they had come across these vessels and were concerned. So they contacted us. »

Wadlow said the Coast Guard passed the reports along to Alaskan Command at Joint Base Elmendorf-Richardson, which said the ships were in the area as part of Russian military drills that some American military officials were aware of. Wadlow said he did not know how big the exercise was or how many Russian ships were taking part.

U.S. military officials have long warned that Russia’s submarine fleet poses a significant threat to national security. Moscow has invested heavily in its submarines, believing they could whittle down U.S. naval superiority in the event of a conflict. China, too, has reportedly been expanding its submarine capabilities for the same reason.

Last month, Lieutenant General Glen VanHerck—the USNORTHCOM and NORAD commander—told a Senate Armed Services Committee hearing: « Russia develops strategic capabilities, such as their submarines, which now are a significant challenge for tracking and pose the potential for cruise missiles that can strike the homeland. »

U.S. Navy Vice Admiral Andrew Lewis said in February that the east coast can no longer be considered a « safe haven » for American ships given Russian submarine activity.