YOKOSUKA NAVAL BASE (AFP) - Britain's top diplomat on Friday urged South Korea to "be bigger" than the North as it renewed propaganda broadcasts through massive loudspeakers at the border in response to Pyongyang's latest nuclear test.

Foreign Secretary Philip Hammond, visiting Japan along with British Defence Secretary Michael Fallon, repeated condemnation of North Korea's actions but questioned Seoul's decision to blast K-pop and criticism of Kim Jong-Un's regime into its territory.

"North Korea acts in a totally irresponsible and provocative way, and I can entirely understand the pressure that the South Koreans feel to respond," Hammond told reporters on a visit to the USS Ronald Reagan, docked at the Yokosuka Naval Base southwest of Tokyo.

"But we have to be bigger than the North Koreans and I would urge South Korea and other like-minded countries in the region to exercise restraint," he added.

"We know that responding in this way is simply rising to the bait that North Korea is presenting to us."

But he said that if South Korea is going to be asked to keep calm, then it is essential that global society come up with proper measures in response.

"Continuing with words is not enough, we have to show we are prepared to take the actions to make the sanctions regime against North Korea effective," he said.

Hammond, who also visited China and the Philippines as part of a swing through the region, noted that Pyongyang ally Beijing "roundly condemned" the test.

"I believe that China is sincere in its desire not to see nuclear capability developed by North Korea," he added.

Fallon and Hammond are in Japan for talks later Friday with their Japanese counterparts and are also expected to meet Prime Minister Shinzo Abe.

The officials earlier visited the Japanese helicopter carrier Izumo - the country's biggest warship since World war II.

"We are both here to show... that Japan is our most important security partner in Asia," Hammond said aboard the Izumo.