US Defense Secretary James Mattis arrived in South Korea on Thursday to discuss security in East Asia amid questions over the new Trump administration's commitment to Washington's regional allies.

Mattis is on a trip that includes a stop in Japan on Friday and Saturday, where he will meet with his Japanese counterpart, Tomomi Inada. In South Korea, he will meet with Prime Minister Hwang Kyo Ahn, who is also acting president, as well as Defense Minister Han Min Koo.

It is the first foreign visit for the new Pentagon chief and also the first of any cabinet member from President Donald Trump's fresh administration. Both Japan and South Korea have been concerned about Trump questioning the value of US alliances on the campaign trail and last week's withdrawal from the Trans Pacific Partnership trade deal.

Those worries have been heightened by the Trump administration getting off to a difficult start with other traditional allies, including Mexico, Australia and Germany.

There are about 28,500 US troops based in South Korea and 47,000 in Japan.

Seoul hopes to get reassurances from the United States that it is still committed to its defense at a time nuclear-armed North Korea remains a threat to regional security.

Pyongyang said last month it was in the final stages of developing an intercontinental ballistic missile, which if fully developed to carry a nuclear warhead could hit US territories. Last week, a US based think-tank said the North appeared to have restarted a nuclear reactor at Yongbyon capable of processing plutonium for a nuclear weapon.

Repeated US and UN sanctions on North Korea over its nuclear and missile programs have failed to curb Pyongyang's ambitions. There is concern the North may test Trump early on by conducting another nuclear explosion or missile test.

A series of atomic and other ballistic missile tests last year prompted the Obama administration to commit to the deployment of the advanced Terminal High Altitude Area Defense (THAAD) system to South Korea.

Watch video 00:26 North Korea 'close to testing' long-range missile

The US and South Korea say the missile defense system is purely defensive against North Korean, but the deployment has angered China. China says the deployment upsets the regional balance of forces at a time Beijing is also flexing its muscles in the South China Sea.

"THAAD is for defense of our ally's people, of our troops who are committed to their defense," Mattis said when he arrived in South Korea. "Were it not for the provocative behavior of North Korea, we would have no need for THAAD out here," Mattis said.

cw/kms (AFP, dpa, Reuters)