New Pentagon chief James Mattis yesterday headed to South Korea and Japan, where he will seek to reassure the key US allies about American security commitments in the region.

The defense secretary's visit marks the first overseas trip of any senior official in the government of President Donald Trump, and comes after the new US leader suggested while campaigning that America's long-standing role in Northeast Asia could change.

“The trip will underscore the commitment of the United States to our enduring alliances with Japan and the Republic of Korea, and further strengthen US-Japan-Republic of Korea security cooperation," the Pentagon said in a statement.

On the campaign trail, Trump raised the possibility of Japan and South Korea arming themselves with nuclear weapons, and accused Seoul of not paying its fair share for the28,500or so US troops stationed on the Korean Peninsula.

Mattis's visit comes amid heightened concerns around North Korea's nuclear

ambitions and a simmering row between Tokyo and Seoul over the use of sex slaves during World War II.

According to South Korea's defense ministry, defense chief Han Min-Koo and his new US counterpart vowedTuesday to push ahead with a plan to deploy a US anti-missile system this year, despite angry protests by China.

The two allies last year announced the plan to deploy the Terminal High Altitude Area Defense (THAAD) system following a series of atomic and missile tests by nuclear-armed North Korea.

The THAAD plan has infuriated China, which fears it will undermine its own ballistic capabilities and has slapped a series of measures seen by Seoul

as economic retaliation.

Mattis will begin his trip in South Korea, where he will meet Han and other senior officials.

Anthony Ruggiero, a North Korea expert from the Foundation for Defense of Democracies, said Mattis could also push for US-South Korea military exercises to go ahead as planned, despite Pyongyang's expected complaints.