North Korea will gain "nothing" by making threats, US President Barack Obama said Friday, warning the isolated state of sanctions with "more bite" if it went ahead with a fourth nuclear test.

Speaking in South Korea as satellite images revealed the North could be preparing for another test, Obama stressed that Washington and Seoul stood "shoulder to shoulder" in their refusal to accept a nuclear North Korea.

"Threats will get North Korea nothing, other than greater isolation," Obama said at a joint press conference with South Korean President Park Geun-hye on Friday.

North Korea-watchers have puzzled over whether the test preparations at the Punggye-ri test site they have seen via satellite images are real, or bravado aimed at stealing the limelight during the US president's tour.

But the latest images suggested increased movement of vehicles and materials near what are believed to be the entrances to two completed test tunnels, the US-Korea Institute at Johns Hopkins University said on its closely followed 38 North website.

Asked how the international community might react, Obama said it would be necessary to look at "additional ways" to apply pressure, including "further sanctions that have even more bite."

In Seoul Friday Obama broached a regional fault line between South Korea and Japan, its two main allies in East Asia, when he said Japan's wartime system of sex slavery "was a terrible, egregious violation of human rights."

"Those women were violated in ways that even in the midst of war were shocking," he said. "They deserve to be heard, they deserve to be respected. And there should be an accurate and clear account of what happened."

South Korea and other nations accuse Japan of failing sufficiently to atone for the forced recruitment of "comfort women" to service its troops before and during World War II.

The issue remains a major irritant in relations between Tokyo and Seoul, and a frustration to Washington, which wants its two major allies in the region to act together against North Korea and forge a united front against a rising China.

"I think [Japanese] Prime Minister [Shinzo] Abe recognizes this and certainly the Japanese people recognize that the past is something that has to be recognized honestly and fairly."

AFP