South Korea says it will join a U.S.-led initiative stop the spread of weapons of mass destruction, after North Korea carried out its second underground nuclear test.

The South Korean Foreign Ministry announced Tuesday that it would join the Proliferation Security Initiative, which began in 2003 to deter states such as North Korea and Iran from trade in missile and nuclear technology.

North Korea has warned it would consider the South's full participation in the program as a declaration of war.

South Korea had planned to join the program following the North's April 9 rocket launch, but delayed the announcement after a surprise offer of a dialogue by the North.