North Korea's official Korean Central News Agency (KCNA), citing a Foreign Ministry spokesman, said on Thursday that Pyongyang would see the siege as "an act of war".
"Should the United States and its followers try to enforce the naval blockade against our country, we will see it as an act of war and respond with merciless self-defensive counter-measures as we have warned repeatedly," the agency said.
The blockade, it added, would be a "wanton violation" of the country's sovereignty and dignity.
The spokesman stated that US President Donald Trump was taking an "extremely dangerous and big step towards the nuclear war" by seeking such a blockade, KCNA said.
The developments came as the US, South Korea and Japan launched rocket tracking drills on Monday aimed at improving detection and monitoring of the North’s ballistic missile tests.
Tensions on the peninsula remain high as concerns grow that a miscalculation during training drills on either side could provoke a full-scale conflict.
Independent analysts in South Korea have recently criticized the proposed blockade, saying it would only worsen the situation and that the US Pacific Command was not in a position to seal North Korea’s maritime borders.
“If the US goes ahead with the blockade, it will only aggravate the situation by provoking North Korea to conduct more nuclear tests and launch missiles under the banner of self-defense,” said Hong Hyun-ik, a senior researcher at the Sejong Institute, this week.
“Instead of searching for a real solution to the nuclear crisis, which is dialogue with the North, the US is attempting to maintain its hegemony in north-east Asia,” he said.
China on Thursday warned against any moves that could trigger a military conflict on the Korean Peninsula, saying the current dispute over North Korea’s weapons program should only be settled through dialogue.
Pyongyang says its nuclear capability is a deterrent against potential foreign aggression. North Korea claims to have successfully tested a new type of intercontinental ballistic missile, topped with a "super-large heavy warhead," which is capable of striking the US mainland.
Trump has vowed to impose additional "major sanctions" against North Korea which is already under multiple sets of penalties.