TOKYO, July 28 (Reuters) - The United States on Sunday announced plans for a major revamp of its military command in Japan to deepen coordination with its ally's forces, as the two countries labelled China the "greatest strategic challenge" facing the region.

The announcement followed security talks in Tokyo between U.S. Secretary of State Antony Blinken and Secretary of Defence Lloyd Austin and their respective Japanese counterparts, Yoko Kamikawa and Minoru Kihara.

"The United States will upgrade the U.S. Forces Japan to a joint force headquarters with expanded missions and operational responsibilities," Austin told reporters after the so-called "2+2" talks.

"This will be the most significant change to U.S Forces Japan since its creation and one of the strongest improvements in our military ties with Japan in 70 years."

The ministers said in a joint statement that the new command structure would be implemented in parallel with Tokyo's own plans to establish a joint command for its forces by March 2025.

The overhaul is among several measures taken to address what the countries said was an "evolving security environment", noting various threats from superpower China.

The statement criticised what it called Beijing's "provocative" behaviour in the South and East China Seas, joint military exercises with Russia and the rapid expansion of its nuclear weapons arsenal.

Beijing's "foreign policy seeks to reshape the international order for its own benefit at the expense of others," the ministers said in their statement.

"Such behaviour is a serious concern to the Alliance and the entire international community and represents the greatest strategic challenge in the Indo-Pacific region and beyond".

China's foreign ministry did not immediately respond to a request for comment from Reuters.

Austin told reporters the command upgrade was "not based on any threat from China" but reflected the allies' desire to work more closely and effectively.

Japan provides a base for the U.S. to project military power in Asia, hosting 54,000 American troops, hundreds of U.S. aircraft and Washington's only forward-deployed aircraft carrier strike group.

Prompted by China's growing military might and regular missile tests by nuclear-armed North Korea, Japan has in recent years shifted dramatically from decades of postwar pacifism. In 2022 it unveiled a plan to double defence spending to 2% of gross domestic product.

The new U.S. command in Japan will be headed by a three-star general, a U.S. official said, however Austin said the U.S. would not rule out appointing a four-star commander to Japan in the future as it has in neighbouring South Korea.

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