South Korea, Japan and the US yesterday held a joint naval exercise aimed at countering missile threats from North Korean submarines, Seoul's defense ministry said, amid mounting concerns over the hermit state's weapons programme.

Pyongyang is on a quest to develop a long-range missile capable of hitting the US mainland with a nuclear warhead, and has so far staged five nuclear tests, two of them last year.

The three-day drills involving more than 800 troops kicked off after US President Donald Trump warned Sunday that the United States is prepared to act unilaterally to deal with North Korea's nuclear program if China proves unwilling to help.

The exercise began off South Korea's southern coast near Japan, featuring multiple naval destroyers and helicopters used in antisubmarine warfare, the ministry said.