Beijing landed a military plane on a disputed South China Sea reef it has built up into an artificial island, officials said Monday, in the first confirmation of such a flight.

An air force plane landed on Fiery Cross reef in the Spratlys archipelago on Sunday to evacuate sick workers, a news report posted online by China's defence ministry said.

China claims nearly all of the strategically vital sea, even waters close to its Southeast Asian neighbours, and has created artificial islands in an effort to assert its claims.

It has significantly expanded Fiery Cross, which is also claimed by Vietnam and the Philippines, drawing international criticism.

In 2014, China began work on a 3,000-metre (10,000 foot) runway on the reef, which is around 1,000 kilometres (620 miles) from its island province of Hainan.

Beijing in January carried out several of what it called civilian flights to Fiery Cross, enraging Hanoi.

"On the Chinese territory, this kind of thing is not surprising at all," foreign ministry spokesman Lu Kang told a regular briefing.

"It is a good tradition of the People's Liberation Army to provide a necessary assistance to Chinese people in need," he added.

This weekend's flight came just days after US Defense Secretary Ashton Carter Friday visited a warship close to flashpoint waters, after announcing joint naval patrols with the Philippines.

On the day of Carter's trip, Beijing said that one of its top military officials had visited a South China Sea island.

Fan Changlong, vice-chairman of the Central Military Commission, observed building work, the defence ministry said, without giving a precise date or location for the visit.

Washington regularly accuses Beijing of militarising the South China Sea, saying it has built runways and deployed weapons to the islands.

Beijing denies the accusations and says US patrols have ramped up tensions.

As well as China, the Philippines, Vietnam, Malaysia, Brunei and Taiwan have overlapping claims to parts of the sea, which are home to some of the world's most important shipping lanes and believed to sit atop vast oil reserves.