South Korea will become a full member of a US-led initiative to curb trade in weapons of mass destruction, the foreign ministry said on Tuesday, one day after North Korea's second nuclear test.
The North has previously said that Seoul's participation in the Proliferation Security Initiative (PSI) would be seen as a declaration of war.
South Korea was previously only an observer in the PSI for fear of offending its communist neighbour.
It decided in principle to become a full member after the North's April 5 rocket launch, but had delayed a formal announcement while trying to resume dialogue.
Government officials quoted by Yonhap news agency said there was no reason to wait any longer following Monday's nuclear test.
The government made its decision "to cope with the serious threat that the proliferation of weapons of mass destruction and missiles poses to world peace and security," said ministry spokesman Moon Tae-Young.
"But maritime agreements agreed between South and North Korea will still remain valid," he said in a terse statement which did not mention the nuclear test.
The initiative, which includes military drills, was launched by then-US President George W Bush in 2003 and now involves more than 90 countries. North Korea has been a leading exporter of missiles in recent years.