The move came amid a growing North Korea-Russia alignment after Russian President Vladimir Putin and North Korean leader Kim Jong-un signed a "comprehensive strategic partnership" treaty that calls for providing military assistance to each other without delay if either side comes under an armed attack, Yonhap news agency reported.

North Korea will "never overlook the moves of the US and its followers to strengthen the military bloc ... but firmly defend the sovereignty, security and interests of the state and peace in the region through offensive and overwhelming countermeasures," North Korea's foreign ministry said in a statement carried by the country's official Korean Central News Agency.

The ministry did not elaborate on what it meant by countermeasures.

On Saturday, South Korea, the US and Japan wrapped up their first, three-day trilateral multi-domain military exercise, titled "Freedom Edge," aimed at strengthening their deterrence against North Korean threats.

The three nations have had combined maritime and aerial exercises before, but Freedom Edge was the first trilateral exercise held across multiple domains, including air, maritime, underwater and cyber.