US Defense Secretary James Mattis plans to travel to Japan and South Korea next month on his first trip as the Pentagon's new chief, a spokesman said Wednesday.

He is set to depart on February 1 for South Korea before traveling to Tokyo on February 3.

"The trip will underscore the commitment of the United States to our enduring alliances to Japan and the Republic of Korea, and further strengthen US/Japan/Republic of Korea security cooperation," Pentagon spokesman Jeff Davis said.

The trip comes amid worries among the two long-standing American allies about the direction of US policy in their region under President Donald Trump.

During his campaign, Trump threatened to withdraw US forces from the two countries if they do not step up their financial support.

Mattis will be the Trump administration's first senior official to shed light on specifics of Washington's intentions in the region.

He will meet with his counterparts and other senior officials, Davis said.

Tensions are also rising with China over its moves to assert sovereignty in disputed areas of the South China Sea, and over North Korean threats to test an intercontinental ballistic missile capable of striking the US mainland.

Trump has angered China with a hard line against Beijing, threatening an economic trade war and upending decades of US policy over Taiwan.

In another jarring note for the region, the president this week withdrew the United States from the Trans-Pacific Partnership, a free-trade agreement signed by former president Barack Obama but never ratified by Congress.

The agreement was signed by 12 countries bordering the Pacific. In addition to the United States, signers include Australia, Brunei, Canada, Chile, Japan, Malaysia, Mexico, New Zealand, Peru, Singapore and Vietnam.

Since his appointment earlier this month, Mattis has sought to reaffirm Washington's major alliances.

On Monday, he emphasized its "unshakeable commitment" to NATO.

During his Senate confirmation hearings, he stressed the US strategic interest in its alliances with Japan and South Korea.

"The United States is stronger when we uphold our treaty obligations," he said.

However, he added, "we expect our allies and partners to uphold their obligations as well."