North Korea, facing international sanction for this week’s nuclear test, threatened on Wednesday to attack the South after Seoul joined a U.S.-led initiative to check vessels suspected of carrying equipment for weapons of mass destruction.

The threat came after South Korean media reported Pyongyang had restarted a plant that makes weapons-grade plutonium.

U.S. President Barack Obama is working to form a united response to Monday’s nuclear test, widely denounced as a major threat to stability that violates U.N. resolutions and brings the reclusive North closer to having a reliable nuclear bomb.

A North Korean army spokesman reiterated that the country was no longer bound by the armistice signed at the end of the 1950-53 Korean War because Washington had ignored its responsibility as a signatory by drawing Seoul into the anti-proliferation effort.

“Any hostile act against our peaceful vessels including search and seizure will be considered an unpardonable infringement on our sovereignty and we will immediately respond with a powerful military strike,” the spokesman for the North’s army was quoted as saying by the official KCNA news agency.

South Korea announced on Tuesday it was joining the naval exercise, called the Proliferation Security Initiative.

An angry Pyongyang, which relies on arms sales for cash, had said it considered such a move an act of war.