White House spokesman Robert Gibbs on Tuesday said President Barack Obama welcomes South Korea's endorsement of the Proliferation Security Initiative.

Mr Gibbs says the United States looks forward to working with South Korea on the project, which tries to stop the spread of dangerous material and weapons and the missiles to deliver them.

Seoul joined the US-led group after North Korea announced it had tested a nuclear weapon on Monday.

North Korea for years has warned the South against joining the blockade. -- AP