TOKYO — Japan and the United States launched their biggest ever joint military exercise Friday amid tensions on the Korean peninsula, the Japanese defence ministry said.

The “Keen Sword” drills, which mark the 50th anniversary of the Japanese-US alliance, will continue until December 10, officials said.

The drill was planned before North Korea’s artillery barrage of a South Korean island last week but comes just days after a US and South Korean show of military force aimed to deter Pyongyang.

Around 34,000 Japanese military personnel with 40 warships and 250 aircraft are joining more than 10,000 US counterparts with 20 warships and 150 aircraft in the drill off Japan’s southern islands, close to the coast of South Korea.

The manoeuvres include integrated air and missile defence, base security, close air support, live-fire training, maritime defence and search and rescue.

The Pacific allies are for the first time being joined by South Korean military observers, in a bid by Tokyo to demonstrate solidarity among the three countries at a time of high tension in the region.

Japan sent soldiers in an observer capacity to take part in joint US-South Korean military exercises in July, held after the sinking of the Cheonan, a 1,200-tonne South Korean naval vessel, the Japanese defence ministry said.

An international probe blamed North Korea for the sinking, which left 46 South Korean sailors dead.

The joint manoeuvres will be much bigger than a naval exercise staged by Washington and Seoul this week in a show of force after Pyongyang stunned the world with the deadly artillery strike on a South Korean border island.

Japan has been on high alert since the attack, with Prime Minister Naoto Kan instructing his ministers to stay in Tokyo during the US-South Korea drill in the Yellow Sea to prepare for any emergencies.

Japan relies heavily on the United States for its security as under its pacifist constitution, its military is not allowed to attack enemy territories.

China’s newly assertive posture on territorial issues this year has also been a cause for concern for Tokyo and other Asian nations in a region where Washington is seen as an important counterbalance.

Following Pyongyang’s November 23 attack on Yeonpyeong island, which killed four people, China has proposed that the six nations involved in long-stalled North Korean denuclearisation talks hold an emergency meeting on the crisis.

But instead the United States, Japan and South Korea have agreed to hold their own talks in Washington on December 6 in an apparent snub to China. The other members of the six-party process are China, North Korea and Russia.