A nuclear attack on the United States or its allies would be met with an "effective and overwhelming" response, US Defense Secretary James Mattis said Friday in South Korea on a trip designed to reassure allies in East Asia.

"Any attack on the United States or our allies will be defeated and any use of nuclear weapons would be met with a response that would be effective and overwhelming," Mattis told reporters.

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, and Prime Minister Shinzo Abe. The defense secretary arrived in South Korea on Thursday for a visit that included meetings 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 new administration. Both Japan and South Korea have been concerned about Trump questioning the value of US alliances on the campaign trail and a decision to withdraw from the Trans Pacific Partnership trade pact.

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.

"America's commitments to defending our allies and to upholding our extended deterrence guarantees remain ironclad," Mattis said at a joint press conference with his South Korean counterpart.

He added the purpose of his trip to South Korea was to "underscore America's priority commitment to our bilateral alliance" and reaffirm its "full commitment" to defending South Korea. In Japan, Mattis struck a similar tone, saying the US stood "shoulder-to-shoulder" with its ally.  

In a first concrete sign the Trump administration would maintain the policies of former President Barack Obama, Mattis and Prime Minister Hwang confirmed they would move ahead with the deployment of the Terminal High Altitude Area Defense (THAAD) system in the South this year.

The United States and South Korea say the missile system is purely defensive against North Korean threats.

The THAAD deployment is strongly opposed by China. "This has not changed and will not change," Chinese Foreign Ministry spokesman Lu Kang said on Friday, adding the US and South Korea should "refrain from going further on this wrong path."

Beijing says THAAD will upset the regional balance of forces by weakening its own nuclear deterrent and ballistic missile capabilities. 

The deployment comes as relations between China and the United States are tense in the South China Sea, where Beijing has set up military bases and other facilities on islands and inlets to assert its sovereignty of the contested waters. The United States has responded by deploying military assets to the region to challenge Chinese claims.

A series of atomic and other ballistic missile tests last year prompted the Obama administration to commit to the deployment of the THAAD system to South Korea. Repeated US and UN sanctions on North Korea over its nuclear and missile programs have failed to curb Pyongyang's nuclear and missile ambitions.

There is concern the North may test Trump early on in his term by conducting another nuclear explosion or missile test.  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.

cw/kms (AFP, dpa)