Japan’s Defence Minister Itsunori Onodera has told his US and South Korean counterparts the threat from North Korea has grown to a “critical and imminent level” and must be addressed.

Mr Onodera’s remarks underscored the deep concern in Tokyo after North Korean weapons tests, including test firing missiles over Japan, as Pyongyang seeks to develop a nuclear-tipped missile capable of reaching the United States.

His comments broke from more measured language on Monday by US Defence Secretary Jim Mattis and South Korean Defence Minister Song Young-Moo, as the three men met on the sidelines of a gathering of Asian defence chiefs in the Philippines.

“(The) threat posed by North Korea has grown to the unprecedented, critical and imminent level,” said Mr Onodera.

“Therefore, we have to take calibrated and different responses to meet with that level of threat.”

South Korea’s Song also acknowledged that “North Korea’s provocative behaviour is becoming worse and worse,” in public remarks before reporters were escorted out of the meeting room.

Mr Mattis renewed sharp criticism of North Korea’s tests, saying they “threaten regional and global security.”

Mr Mattis, who kicked off a week-long trip to the region on Monday, has been eager to emphasise diplomatic efforts to resolve the crisis peacefully as escalating tension between Washington and Pyongyang stoked fears of ‘armed confrontation.’

Asked about his conversation with Mr Onodera after the two met earlier in the day, before joining South Korea’s Song, Mr Mattis said they discussed “maintaining stability and peace in support of the diplomats.”

Meanwhile, former US President Jimmy Carter said he is willing to travel to North Korea on behalf of the Trump administration to help diffuse the situation, the New York Times reported.

– AAP