Japan and South Korea must cooperate to counter any threat from North Korea, Japan's new prime minister, Yoshihide Suga, told South Korea's president on Thursday, calling for the uneasy neighbours to repair their frayed ties.

Relations between the two U.S. allies have deteriorated sharply over the past year over war-time history and trade, in particular the issue of Korean labourers forced to work at Japanese firms during Japan's 1910-1945 colonial rule.

"I told President Moon that we cannot leave our current very difficult relations where they are now," Suga told reporters after a telephone call with South Korea's President Moon Jae-in.

"Cooperation between Japan and South Korea, as well as between Japan, the United States and South Korea, is crucial to deal with North Korea and other issues."

Japan would "continue to firmly seek appropriate action from South Korea", he said. He did not elaborate.

South Korea had asked for the telephone call, Japan said.

In a letter to Suga last week, Moon had said he was willing to sit down any time to improve ties.

Suga replaced Shinzo Abe as prime minister last week.