North Korea has sent a stern warning to South Korea following Seoul’s decision to fully participate in the U.S.-led Proliferation Security Initiative (PSI).

The statement, issued by the North’s permanent military mission to the Joint Security Area at the DMZ on Wednesday, said Pyongyang will respond militarily to any attempts by South Korea to interdict or inspect North Korean ships, as it will regard such moves as violations of North Korean sovereignty.

Officials from the mission also emphasized that Pyongyang can no longer guarantee the safety of U.S. and South Korean naval vessels and other commercial vessels sailing along the demarcation line between the two Koreas in the Yellow Sea.

The South Korean government on Tuesday announced its full participation in the PSI, a day after the North conducted a nuclear test. The North had said in the past that a decision by Seoul to fully participate in the PSI would be considered a declaration of war.

The PSI aims to halt the spread of weapons of mass destruction and related technologies by searching ships in international waters suspected of transporting such materials.