BEIJING (AP) — China is holding military exercises in the disputed South China Sea coinciding with U.S. President Joe Biden’s visits to South Korea and Japan that are largely focused on countering the perceived threat from Beijing.

China’s Maritime Safety Administration office in the southern island province of Hainan said the drills began Thursday and will continue through Monday.

It said other aircraft and vessels will be prohibited from entering the area but gave no further details. China claims virtually all of the South China Sea and the crucial waterway has become a potential flashpoint for conflict in Asia.

The U.S. does not take a position on the sovereignty issue but insists on the right to operate freely in the sea and frequently sails warships close to militarized Chinese-held islands in the area in what are termed freedom of navigation operations.

China routinely protests such missions, labeling them deliberate provocations that endanger peace and stability. To assert its claim, it has built airstrips and other military infrastructure atop human-made islands built on coral reefs and atolls.

Brunei, Malaysia, the Philippines and Taiwan also exercise overlapping claims in the South China Sea. The Philippine coast guard said Friday it had established outposts on three islands in the contested waters, a move that will likely be frowned upon by Beijing,

Since the beginning of the month, China’s first aircraft carrier, the Liaoning, has been conducting a mission in the Sea of Japan. The Defense Ministry described it as “routine training” aimed at boosting performance that is “in line with relevant international law and international practice, and not targeting any party.”

China also flew a pair of long-range nuclear-capable H-6 bombers through the area on Wednesday, Chinese media reports said.

Meanwhile Friday, the aircraft carrier USS Ronald Reagan and its strike group left their home port at Yokosuka, Japan, for a “routine at-sea period,” the 7th Fleet said.

The ships and the carrier air wing “are expected to work with allies and partners, promote adherence to a rules-based international order, as well as maintain presence and flexibility to meet the needs of the U.S. Department of Defense,” the Navy said.

Biden arrived in South Korea on Friday. While in Japan, he will meet Tuesday with fellow leaders of the Indo-Pacific strategic alliance known as the Quad, a group that includes Australia, India and Japan.

The four nations share concerns over China’s growing regional assertiveness and increasingly capable armed forces.

China views the grouping as a part of a U.S.-led push to impede its economic and political rise and frustrate its attempts to intimidate self-governing Taiwan into accepting its demand to unify with the mainland.

On Wednesday, Chinese Foreign Minister Wang Yi criticized what he called negative moves by Washington and Tokyo against Beijing during a video call with Japanese Foreign Minister Yoshimasa Hayashi.

“What arouses attention and vigilance is the fact that, even before the American leader has set out for the meeting, the so-called joint Japan-U.S. anti-China rhetoric is already kicking up dust,” Wang said, according to China’s Foreign Ministry.

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