China said it had landed a test flight on a newly completed airfield in the disputed Spratly Islands in the South China Sea, a sign of its growing military capabilities in the region.

The flight drew a quick protest from Vietnam, which said China had “seriously violated” its sovereignty.

According to a statement released late Saturday by the Chinese foreign ministry, Beijing has completed construction of an airfield on Fiery Cross Reef, and recently used a civil aircraft to conduct a flight testing whether the facilities were up to civil-aviation standards.

Vietnam said Saturday that it had lodged an official protest with the Chinese Embassy in Hanoi over the action. The Spratly Islands are the subject of overlapping claims by several neighbors, including the Philippines, a U.S. ally.

The test flight came after a visit to Hanoi by Chinese President Xi Jinping in November, which analysts say was partly designed to address the souring relationship between the two countries over territorial disputes.

In the statement posted on the Vietnamese foreign ministry’s website Saturday, ministry spokesman Le Hai Binh said China’s action went against “the common conception of the high-ranking leaders of the two countries, and against an agreement on basic principles for solving maritime issues between Vietnam and China.”

With Beijing’s latest acknowledgment, China has now completed two airfields in the South China Sea, said Andrew Erickson, an associate professor at the U.S. Naval War College. Apart from the Spratly Islands airfield, Beijing also has an operational airfield on Woody Island, which is located southeast of China’s Hainan Island and is a part of the Paracel island chain, also claimed by Vietnam.

With a length of around three kilometers, the completed airfield is a sign of China’s ability to flex its military power in the region, he said. “While this was a civil test, this airport is clearly very militarily capable, and China could presumably start to use it in some capacity at any time,” said Mr. Erickson.

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