SEOUL (REUTERS, AFP) - US and South Korean defence chiefs have agreed to deploy a US missile defence system in South Korea this year to counter the threat from North Korea, South Korea's defence ministry said on Friday (Feb 3).

US Defence Secretary James Mattis is on his two-day visit to Seoul, meeting South Korean top officials including Defence Minister Han Min Koo.

"The two agreed to deploy and operate Terminal High Altitude Area Defence (Thaad) system within this year as planned, which is a defence system solely against North Korea's missile threat," the South Korean defence ministry said in a statement, referring to the meeting between Mr Han and Gen Mattis.

The US defence chief arrived in South Korea on Thursday on the first leg of a trip that also includes Japan, two key allies rattled by US President Donald Trump's ascent to power.

It is the first overseas tour by a senior official in Trump's administration as concerns rise about the direction of American policy in the region under the protectionist and fiery leader.

On the campaign trail, Mr Trump threatened to withdraw US forces from the two countries if they do not step up their financial support. Some 28,500 US troops are based in South Korea and 47,000 in Japan.

The rhetoric has raised concerns in both Seoul and Tokyo, and in a statement this week, South Korea's defence ministry said it hoped Gen Mattis' trip would be "an opportunity for the Trump administration to maintain and strengthen US commitment" to their alliance.

"President Trump ... wanted me to make a very clear statement about the priority that we place on this alliance between our two allies", Gen Mattis said at the start of his talks with Mr Hwang Kyo Ahn, who serves as the South's acting leader following the impeachment of President Park Geun Hye in December over corruption charges.

The two allies last year announced plans to deploy the Thaad system following a series of North Korean atomic and missile tests, infuriating China which fears it will undermine its own ballistic capabilities.

North Korean leader Kim Jong Un boasted last month that the country was in the "final stages" of developing an intercontinental ballistic missile.

But it remains unclear whether the missile shield system would actually be deployed in South Korea after the country's leading presidential candidate and opposition member Moon Jae In argued the decision should be left to the next government.

South Korea may hold elections in a matter of months if the Constitutional Court signs off on Ms Park's impeachment.

Should it reject the impeachment and restore her to presidency, the election would take place in December.

The prime minister said the fact that South Korea was the first country for an overseas visit by Gen Mattis underscored the "importance" Washington put in the alliance between the two countries.

Japanese Prime Minister Shinzo Abe - who is scheduled to meet Mr Trump next week in Washington - told lawmakers he intends to press Gen Mattis about "the significance of the Japan-US alliance".

The latter's Asia tour comes as relations between the US and other world powers such as Mexico and Australia get off to a rocky start.

The Washington Post reported late Wednesday that Mr Trump ripped into his Australian counterpart Malcolm Turnbull during a call last week, with the US president apparently fuming at a refugee accord he called "dumb" and cutting the call short.

Australia is a close US ally, and one of the so-called "Five Eyes" countries with which the US routinely shares sensitive intelligence.

Mr Trump has meanwhile angered Mexicans by ordering the construction of a massive border wall and vowing to make their country pay for it.