BEIJING (AFP/REUTERS) - China has expanded its presence in the contested South China Sea by switching on a lighthouse atop a reconstructed reef also claimed by Vietnam and the Philippines, state media said.

Chinese transport officials held a ceremony on Subi on Tuesday (April 5), Xinhua news agency said. Pictures showed men in white shirts beside the towering structure next to a sign reading "Lights-on ceremony".

The 55m-high facility on Subi Reef in the Spratly chain contains technology to monitor passing ships, the official Xinhua reported late on Tuesday.

Asked about the lighthouse, Foreign Ministry spokesman Lu Kang said China was dedicated to providing public services in the South China Sea to ensure safety and freedom of navigation, which would be helpful for commercial users of the waters.

Xinhua said the lighthouse, which emits a white light at night, "can provide efficient navigation services such as positioning reference, route guidance and navigation safety information to ships, which can improve navigation management and emergency response".

The South China Sea is an important maritime area and major fishing ground, it added.

"However, high traffic density, complex navigation condition, severe shortage in aids and response forces have combined to threaten navigation safety and hindered economic and social development in the region."

China has lighthouse projects on two other reefs in the area - Cuarteron Reef and Johnson South Reef.

China claims virtually all the South China Sea despite conflicting claims by Brunei, Malaysia, Vietnam, Taiwan and the Philippines. It has constructed artificial islands in the area in recent months as it asserts its claims.

China has turned Subi Reef - known as Zhubi in China - into an artificial island in the past year, satellite photos have shown, reclaiming nearly 400ha of land.

Before China turned it into an island, Subi was submerged at high tide.

Under the UN Convention on the Law of the Sea, 12-nautical-mile limits cannot be set around man-made islands built on previously submerged reefs.

China says much of its construction in the South China Sea is designed to fulfil its international obligations in terms of maritime safety, search and rescue and scientific research.

The island-building has been condemned by neighbours and the United States, but Beijing insists it is aimed at helping with maritime search and rescue.

Washington regularly accuses Beijing - which says it has built runways and deployed unspecified weapons to the islands - of militarising the area.

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

Citing an obligation to uphold freedom of navigation, Washington last year sent the USS Lassen to sail past Subi Reef, a move which angered Beijing.

Xinhua reported last year that China would build two 50m-tall lighthouses on the Cuarteron and Johnson South reefs in the Spratly islands, which are also claimed by Vietnam and the Philippines.

The state-run China Daily newspaper reported in 2014 that Beijing would build five new lighthouses in the South China Sea's Paracels chain.