US Secretary of Defense Lloyd Austin has stressed the importance of joint efforts among Japan, the United States and Australia to upgrade the operational capabilities of their forces.

Austin, who is visiting Japan, on Wednesday observed a trilateral command post drill involving Japan's Ground Self-Defense Force and the US and Australian militaries.

The joint drill began on December 2 at GSDF Camp Asaka, which straddles Tokyo and its northern neighbor Saitama Prefecture.

Austin told those taking part: "This is a picture of jointness and also combined operations as well. This is... how we ought to be operating, and it's really good to see working hand and glove together and taking us to the next level."

On Tuesday, Austin met Japanese Prime Minister Ishiba Shigeru. They agreed to continue efforts to enhance command and control of the Self-Defense Forces and US forces and promote technological cooperation on defense equipment, under the Japan-US alliance.

Austin will step down in January when US President-elect Donald Trump takes office. His Tokyo visit appears to be aimed at maintaining a stable Japan-US alliance following the change of power in Washington.