The announcement that Seoul would join the Proliferation Security Initiative (PSI) was originally set for Wednesday.

North Korea, a leading exporter of missiles in recent years, has warned that Seoul's participation would be tantamount to a declaration of a war.

Media reports and an aide said President Lee Myung-Bak has grown cautious about the issue, following Pyongyang's furious response to a UN Security Council statement censuring its April 5 rocket launch.

The North has announced it is quitting six-nation nuclear disarmament talks and will restart the plants which produce weapons-grade plutonium.

"President Lee's underlying attitude is to be cautious when handling the sensitive PSI issue," an aide told AFP on condition of anonymity.

The aide refused to elaborate on why the announcement was postponed or whether Seoul has changed its decision to play a full part in the PSI.

Under previous governments Seoul was only an observer in the drill aimed at stopping ships and aircraft, for fear of souring relations with its communist neighbour.

DongA Ilbo newspaper, quoting a presidential source, said Lee had questioned if this was the right time for the announcement.

"The government has not changed its decision to participate in the PSI. President Lee just wants to choose the date of the announcement more carefully," the source said.

Chosun Ilbo newspaper said hawks from the foreign ministry want an early announcement, while doves -- mainly from the unification ministry, which handles cross-border relations -- want a postponement.

Wednesday was the North's biggest holiday, the 97th anniversary of its founder's birth.

The fate of a South Korean worker detained since March 30 at the Seoul-funded Kaesong industrial estate could also complicate the issue.

The North claims he criticised its regime and tried to persuade a local woman worker to defect, but has blocked access to the employee.

Foreign Minister Yu Myung-Hwan has repeatedly said Seoul will play a full part in PSI. His deputies say Seoul is consulting other countries.

China suggested Wednesday the time was not right for Seoul to sign up.

"The current situation is already too complex and China doesn't want it to become further complicated," its ambassador to Seoul, Cheng Yonghua, told a lecture when asked how Beijing views Seoul's move to join the PSI.

"We hope for respective parties to maintain positive and active attitudes and believe that tension should not be heightened," Yonhap news agency quoted Cheng as saying.