US President Joe Biden heads to Japan today, the second leg of his trip, which aims to strengthen US leadership in Asia in the face of the rise of China's influence and the nuclear threat posed by North Korea.

Biden is leaving South Korea, where he held a series of meetings with newly elected President Yoon Suk-Yul, during which they discussed the issue of expanded military exercises in the face of threats from North Korean leader Kim Jong Un.

At a time when officials from both countries warned that Kim could raise tension by testing a nuclear weapon while Biden was in the region, the US president said allied democracies should deepen ties.

At a joint news conference with Yoon, Biden spoke of a "global competition between democracies and authoritarian regimes," saying the Asia-Pacific region is a major battleground.

While China is the main rival of the United States in this conflict, Biden highlighted the challenge posed by Russia when he signed a $40 billion aid package late Saturday to help Ukraine fight Russian forces.

In Japan, Biden will meet Prime Minister Fumio Kishida and Emperor Naruhito on Monday before a four-way summit on Tuesday that brings together the leaders of Australia, India, Japan and the United States.

And Biden will unveil - also Monday - a new major US initiative for regional trade, which his country considers the economic framework for the prosperity of the Indo-Pacific region.

In a statement issued on Saturday, Biden and Yoon said that in view of the "persistent threat from North Korea," they "agreed to begin discussions to expand the scope and scale of joint military exercises on and around the Korean Peninsula."

The potential boost to joint military exercises between the United States and South Korea is in response to North Korea's nuclear tests this year.