U.S. Secretary of Defense Jim Mattis says the fate of U.S. Forces Korea is not an issue to discuss with North Korea.  

Mattis, who is attending the Asia Security Summit in Singapore, said the issue is not on the table for the North Korea-U.S., nor should it be.

He also vowed efforts to achieve complete, verifiable and irreversible denuclearization(CVID) of the Korean Peninsula.

During a speech at the security summit on Saturday, he said the U.S. will support unrelenting diplomatic efforts for this aim.

South Korean Defense Minister Song Young-moo also spoke at the summit and said the South will not pursue North Korea's collapse or Korean unification through absorption or any artificial means, and vowed to gradually ease inter-Korean military tensions. 

The Moon Jae-in administration, he said, acknowledges the differences that exist between the two Koreas. 

Song will also hold talks with Mattis and discuss security measures related to denuclearization and defense cooperation.

On Sunday, a trilateral meeting with the Japanese and Chinese defense ministers is scheduled.

The Asia Security Summit is also known as the Shangri-La Dialogue as it is held annually at the Shangri-La Hotel in Singapore.

It brings together senior defense officials and security experts from around 40 nations.