US Defense Secretary Mark Esper met with senior South Korean leaders on Friday, amid a series of regional challenges ranging from tensions between Seoul and Tokyo to how much of the cost South Korea should pay for American troops stationed there.

Esper, on his first international trip since being confirmed as defense secretary, arrived in South Korea on Thursday evening amid an escalating feud between Seoul and Tokyo, both US allies.

South Korea has said it was exploring all options in a bitter trade dispute with Japan, including scrapping an intelligence sharing pact.

Esper arrived in Seoul a day after President Donald Trump said South Korea agreed to "pay a lot more" to shoulder the costs required for the upkeep of 28,500 US troops in South Korea and that talks are under way to discuss the issue.

But a spokesman at South Korea's foreign ministry told reporters on Thursday that the negotiations have not yet begun.

On Friday, South Korean President Moon Jae-in appointed Lee Soo-hyuck as the new ambassador to the US who has once described Donald Trump as "treacherous."