Beijing anger as Philippines repairs rusty wartime ship serving as South China Sea outpost

The Philippines said on Wednesday it was repairing a crumbling ship serving as its lonely outpost in the disputed South China Sea as China deploys more vessels and builds new islands nearby.

This would ensure the rust-eaten second world war vintage BRP Sierra Madre remains livable for a tiny unit of marines guarding Second Thomas Shoal, said Philippine navy spokesman Colonel Edgard Arevalo.

"Maintenance repair is being done to ensure the vessel's minimum habitability. We are morally and duty-bound to provide for our troops there," he said.

Beijing condemned the Philippines for carrying out the repairs, demanding Manila remove the vessel.

The Foreign Ministry said Beijing "reserved the right to take further measures", without elaborating.

The Philippine military deliberately grounded the 100-metre vessel atop the reefs in 1999 in a last-ditch effort to check the advance of China, which four years earlier occupied Philippine-claimed Mischief Reef some 40km away.

The disputed outcrops are located around 200km from the western Philippine island of Palawan and roughly 1,100km from the nearest major Chinese land mass.

The Philippines regularly rotates a group of around nine marines aboard the former tank-landing ship, which first saw service for the United States navy in the second world war. It was acquired by the Philippine navy in the 1970s.

In recent months the Philippine military has accused China of employing its maritime surveillance ships to harass the former's resupply missions to the Sierra Madre.

In an incident witnessed by Filipino journalists, a Chinese coast guard ship attempted to block a Philippine vessel that was to bring fresh supplies to Filipino troops on the Sierra Madre in March last year.

Arevalo said the repairs were not a response to China's island-building.

Arevalo said he could not confirm reports of a plan by the military to build a helipad on the ship.

The Philippines has asked a United Nations-backed tribunal to declare China's claim over most of the resource-rich South China Sea as illegal. China has refused to participate.

Vietnam, Taiwan, Malaysia and Brunei also have claims.