``Secretary Cohen reaffirmed the US commitment to render prompt and effective assistance to the RoK (South Korea) for repelling any armed attack against the RoK in accordance with the RoK-US mutual defense treaty and to provide a nuclear umbrella for the RoK,'' a joint South Korea-US communique said.

The communique was released following meeting between Cohen and his South Korean counterpart Chun Yong-Taek.

The defence leaders agreed in their talks that suspicions over a North Korean underground facility needed to be clarified.

The US suspects North Korea is using the site to revive its nuclear programme while the Communist state has said it was being used for undisclosed civilian purposes.

``With regard to the suspect underground construction inNorth Korea, the two ministers agreed that the nature of this construction should be clarified as soon as possible,'' the communique said.