Story highlights A Chinese military jet's actions in the South Pacific concern the U.S. military

The dispute involves Chinese-built man-made islands

China says it was a rescue mission

Washington (CNN) China's apparent landing of a military jet on a man-made island in the disputed waters of the South China Sea drew a protest from the U.S. military Monday.

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The Chinese military aircraft landed on Yongshu Reef, also known as Fiery Cross Reef, to give emergency assistance to three severely ill civilian workers, said Lu Kang, spokesman with China's Ministry of Foreign Affairs. The three construction workers were airlifted Sunday to a hospital on Hainan island, according to Xinhua, the Chinese state-run news agency.

But U.S. officials had some questions.

"We're aware that a Chinese military aircraft landed at Fiery Cross Reef on Sunday in what China described as a humanitarian operation to evacuate three ill workers," Pentagon spokesman Capt. Jeff Davis told CNN in a statement. "It is unclear why the Chinese used a military aircraft, as opposed to a civilian one."

The three patients were transported to a hospital on Hainan island, according to state-run media.

The United States, along with the Philippines have voiced continued concern over China's establishment of man-made islands in portions of the South China Sea, one of the world's busiest sea lanes for commerce, in areas claimed by the Philippines and thousands of miles from the Chinese mainland.

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