South Korea will not join a US-led scheme to stop and search suspicious North Korean ships, officials say.
South Korea has been wary of searching North Korean ships
Seoul has been under increased pressure to join the Proliferation Security Initiative (PSI) since North Korea conducted a nuclear test on 9 October.
Officials said South Korea did not want to antagonise the North as it had moved to rejoin six-party nuclear talks.
The announcement came as nations finalise reports to the UN on imposing sanctions against North Korea.
South Korea's Deputy Foreign Minister Park In-kook told reporters that South Korea supported the "purpose and principles" of the PSI.
But he said South Korea would not be formally joining it "in consideration of special circumstances on the Korean peninsula".
South Korean officials are believed to be worried that intercepting a North Korean ship could trigger an armed clash, which is of particular concern since the two countries remain technically at war.
But South Korea's decision will be seen as a set-back to US efforts to present a united front against North Korea.
Mr Park said South Korea's decision could change depending on the progress on the six-nation talks, which are due to restart within the coming weeks.
The issue of North Korea is set to dominate when 21 world leaders meet in Vietnam this weekend for the Asia-Pacific Economic Cooperation (Apec) forum.
At the same news conference, Lee Kwan-se, a Unification Ministry official, said South Korea would continue with the measures it introduced even before the October test - including the suspension of humanitarian aid to the North.
South Korea is one of several countries due to submit their report to the UN on Monday, outlining how they will implement the sanctions, which call for an arms embargo, a travel ban for senior officials and a ban on the shipment of luxury goods.
The PSI is a voluntary initiative that was launched in 2003 as a way of stopping the spread of weapons of mass destruction by stopping suspicious cargo ships at sea.