South Korea feared that any attempt to stop and check suspicious ships near North Korea as part of UN sanctions would spark an armed clash, a senior official said on Friday.
Seoul is being urged by the US to play a greater role in an international initiative to inspect cargos following the North's declared nuclear test, but is fearful of sparking a war with its communist neighbour.
"The possibility of sparking an armed clash would be very high," said Vice Foreign Minister Yu Myung-Hwan, referring to any searches off the Korean Peninsula.
"That's why we don't take part in PSI," he was quoted by Yonhap news agency as telling legislators, referring to the Proliferation Security Initiative introduced in 2003.
PSI is a US initiative calling for the interdiction of vessels and airliners suspected of carrying weapons of mass destruction or related materials.
It would be expanded under UN sanctions imposed on North Korea following its October 9 declared nuclear test. The sanctions resolution provides for the inspection of cargos to and from North Korea.
Yu said consultations were underway among various government agencies to discuss ways to expand South Korea's role in PSI, short of taking part in such searches off Korea. It currently has observer status.
Song Min-Soon, top security adviser to President Roh Moo-Hyun, said South Korea was considering ways to expand its roles in PSI "with a position that we do not take part in sea blockades against North Korea".
The US has denied the ship searches would amount to blockades.
US ambassador Alexander Vershbow said earlier Friday he understood the Seoul government was considering closer involvement in PSI.
"I expect that the ... government, after the healthy democratic debate on the issue that is now underway, will take appropriate steps to signal its resolve that North Korea's behaviour is unacceptable," he said.
Multinational PSI exercises in the past have seen high-speed maritime chases and commandos rappelling onto vessels from helicopters or clambering aboard from fast boats, with inspectors in chemical suits searching suspect cargo.