China on Tuesday once again rejected US concerns over a military aircraft sent by the People's Liberation Army (PLA) for the evacuation of sick workers from Nansha Islands, stressing indisputable sovereignty rights over the islands and its neighboring waters that the US has no right to comment on.

Serving the people whole-heartedly is the mission of the PLA and rescuing the people in danger is a fine tradition of the PLA, China's defense ministry said on Tuesday, referring to a rescue mission on Sunday where a military aircraft patrolling over the South China Sea received orders to evacuate three seriously ill workers at a construction site on Yongshu Reef.

CNN quoted Pentagon spokesman Captain Jeff Davis as saying it was unclear why China had used a military aircraft rather than a civilian one in the landing, Reuters reported.

The defense ministry also expressed doubt s whether the US military would stand aloof if US citizens fell suddenly ill on US soil.

Chinese foreign ministry spokesperson Hua Chunying also said China was "baffled" by Pentagon's remarks.

"The participation of military planes in disaster relief, humanitarian assistance and evacuating civilians is a common practice around the world ... On Chinese soil, Chinese military planes and personnel will show up whenever they are needed," Hua said at a Tuesday press conference.

China began to reclaim land on Yongshu Reef in August 2014. The US has repeatedly criticized construction on the islands, and has expressed concern that China plans to use them for military purposes.

China has indisputable sovereignty over the Nansha Islands and the US has no right to point fingers at China's building activity and defensive facilities there, the defense ministry stressed.

Hua questioned why the US sends military ships and planes, instead of civilian ones, to what they described as efforts to maintain freedom of navigation in the South China Sea, while urging the US to take real actions to safeguard peace and stability on the sea.