North Korea threatened on Wednesday to attack the South after it joined a US-led plan to track and search ships suspected of carrying equipment for weapons of mass destruction.
On Wednesday the communist country fired another missile, the latest of several launched since its underground nuclear test on Monday.
South Korean media reported that Pyongyang had also restarted a plant that makes plutonium that can be used in nuclear bombs.
The United Nations Security Council is discussing ways to punish North Korea for Monday's test, which has drawn international condemnation. The test is viewed as a major threat to regional stability and brings the reclusive North closer to having a reliable nuclear bomb.
North Korea's latest threat comes after Seoul announced it would join the US-led Proliferation Security Initiative, launched under the George W Bush administration as a part of its "war on terror".
The state-run news agency KCNA quoted an army spokesperson as saying the North would launch a powerful military strike if there was any "hostile" acts against its ships.
The army spokesperson said the North was no longer bound by an armistice signed at the end of the 1950-53 Korean War, because the United States had ignored its responsibility as a signatory by drawing Seoul into the anti-proliferation effort.
Monday's nuclear test has raised concern about Pyongyang spreading weapons to other countries or groups. The US has accused it of trying try to sell nuclear know-how to Syria and others.
The rival Koreas fought two deadly naval clashes in 1999 and 2002 near a disputed maritime border off their west coast and the North has threatened in the past year to strike South Korean vessels in those Yellow Sea waters.