The neighbours are embroiled in a trade row stemming from long-running disputes over wartime history, and Seoul announced in August it would terminate the General Security of Military Agreement (GSOMIA) when it expires next week.

"GSOMIA is an important tool by which Korea, the United States and Japan share effective and timely information, particularly in times of war," Esper said in Seoul, where he was beginning an Asian tour.

"The only ones who benefit from expiration of GSOMIA and continued friction between Seoul and Tokyo are Pyongyang and Beijing," he told reporters.

"We urge all sides to sit down and work through their differences."

Seoul and Tokyo are both major US allies, democracies and market economies faced with an overbearing China and wayward, nuclear-armed North Korea.

But their relationship is heavily coloured by territorial and historical disputes stemming from Japan's bitterly-resented 35-year colonial rule over the peninsula in the early 20th century, including the use of wartime sex slaves and forced labour.

The current tit-for-tat row was triggered by South Korean court rulings against Japanese companies and has seen the neighbours remove each other from their lists of trusted trade partners.

Speaking alongside Esper at a joint press conference, his South Korean counterpart Jeong Kyeong-doo blamed Tokyo for Seoul's decision to end the agreement.

"Japan imposed trade restrictions and removed South Korea from its white list, saying it can't trust Seoul for issues surrounding security," he said.

Seoul had urged Washington to make "active efforts" to persuade Tokyo to settle the trade issue before it could reconsider the GSOMIA decision, he said.

The US stations 28,500 troops in the South to defend it against the North, which invaded in 1950, triggering the Korean war, and the Trump administration is demanding Seoul increase its contributions to their costs.

A rise was "crucial", Esper said, saying the South "is a wealthy country and could and should pay more to offset the cost of defence".

The US is reportedly demanding a fivefold increase in the payments to nearly $5 billion a year.

Seoul has baulked at the figure and Jeong said that burden-sharing had been "fair and reasonable" so far and should remain so, albeit at a level that would "improve" the alliance.