Tension on the Korean peninsula has escalated after North Korea said it no longer considers itself bound by the terms of the 1953 armistice between the two Koreas and threatened to use military force if provoked.

The announcement came as South Korea joined a US-led initiative to check vessels suspected of carrying equipment for weapons of mass destruction.

The Democratic People’s Republic of Korea (DPRK), aka North Korea, is the subject of global condemnation and calls for greater sanctions after conducting an underground nuclear test and launching up to five medium-range ballistic missiles earlier this week.

South Korea’s largest newspaper, Chosun Ilbo, said the repressive state appears to have made good on a threat issued in April of restarting a facility at its Yongbyon nuclear plant that extracts weapons-grade plutonium.

Quoting an unnamed government source, the newspaper said, “there are various indications that reprocessing facilities in Yongbyon resumed operation [and] have been detected by [a] US surveillance satellite, and these including steam coming out of the facility.”

Pyongyang said it considers Seoul’s decision to join the US-led naval blockade as tantamount to a declaration of war.

A North Korean army spokesman was quoted by the official Chosun Central News Agency (KCNA) as saying, “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.”

US 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 UN resolutions and brings the Stalinist-state closer to having a reliable nuclear bomb.

The refusal of North Korea to participate in UN sponsored elections resulted in the Korean War between 1950 and 1953. Though an armistice was signed in 1953 the two countries are officially still at war as a peace treaty was never signed.