The United States, South Korea and Japan will hold joint naval exercises next week in waters around the Korean peninsula, a US defence official said Thursday, as fears grow over Pyongyang's nuclear programme.

The announcement comes as US Secretary of Defense Chuck Hagel and Secretary of State John Kerry were in Tokyo to meet their Japanese counterparts following talks with South Korean officials this week.

The US defence official gave few details about the three-nation drills, calling them "increasingly common" and "designed to strengthen coordination and improve readiness to respond to situations such as humanitarian assistance and disaster relief".

"Next week, in waters around the Korean peninsula, the USS George Washington Strike Group will conduct a trilateral maritime exercise with the Republic of Korea Navy and the Japan Maritime Self-Defense Force," he said.

The announcement was made a day after the United States and South Korea said they had mapped out a new strategy to counter the growing threat of a North Korean nuclear or chemical weapons attack.

Observers said the new plan was largely a confidence-building measure to underline Washington's support for Seoul against any provocation from Pyongyang.

The naval drills also come as US President Barack Obama seeks to rebalance US diplomatic and military weight towards the rising Pacific region.

Aside from tensions over North Korea, Japan is embroiled in a bitter sovereignty row with Beijing over ownership of islands in the East China Sea, which analysts have warned could be the flashpoint for a military conflict between the two Asian giants.

South Korea and Japan are themselves involved in a dispute tied to long-held territorial claims and anger over Tokyo's wartime aggression.

North's Korea's nuclear test in February -- its third and most powerful to date -- triggered months of heightened military tensions on the Korean peninsula.

Pyongyang threatened pre-emptive nuclear strikes against South Korea and the United States, while the Pentagon responded by deploying nuclear-capable B2 stealth bombers during joint military exercises with Seoul.

And in March, Seoul and Washington signed another pact providing for a joint military response even to low-level provocation by North Korea.

Tensions have since eased, but acute concerns remain over the North's nuclear programme, with a US think-tank saying Wednesday that Pyongyang has clearly restarted an ageing plutonium reactor, after analysing new satellite imagery.

The US-Korea Institute at Johns Hopkins University, which last month reported signs that the Yongbyon reactor had resumed operation, said that more conclusive evidence had emerged in the latest image.