BEIJING - China condemned on Friday what it called the "illegal occupation" of a disputed coral reef by the Philippines, and vowed to protect its sovereignty after Manila moved new soldiers and supplies to the remote location.

The Second Thomas Shoal, known in China as the Ren'ai reef, is at the centre of the latest territorial dispute between Beijing and Manila. Both countries have been locked in a decades-old territorial squabble over the South China Sea.

The Philippines is accusing China of encroachment after three Chinese ships, including a naval frigate, converged just 5 nautical miles (9 km) from an old transport ship that Manila ran aground on a reef in 1999 to mark its territory.

"China's determination to safeguard its national sovereignty is resolute and unwavering and (we) will never accept any form of illegal occupation of the Ren'ai reef by the Philippines," Chinese Foreign Ministry spokeswoman Hua Chunying told a regular briefing.

Philippine armed forces chief General Emmanuel Bautista told reporters on Wednesday the military had brought in a fresh team to replace soldiers stationed on the wrecked ship on the reef and replenished their supplies, including food, water and fuel.

China's claims over islands, reefs and atolls in resource-rich waters off its south coast and to the east of mainland Southeast Asia have set it directly against Vietnam and the Philippines, while Brunei, Taiwan and Malaysia also lay claim to parts.

The Second Thomas Shoal, a strategic gateway to Reed Bank, believed to be rich in oil and natural gas, is one of several possible flashpoints in the South China Sea that could force the United States to intervene in defence of its Southeast Asian allies.

In 2010, Manila awarded an Anglo-Filipino consortium a licence to explore for gas on Reed Bank, but drilling stalled last year, because of the presence of Chinese ships.

Manila says Reed Bank, about 80 nautical miles (148 km) west of Palawan island at the southwestern end of the Philippine archipelago, is within the country's 200-nautical mile (370 km) exclusive economic zone.

Beijing says it is part of the Spratlys, a group of 250 uninhabitable islets spread over 165,000 square miles, claimed entirely by China, Taiwan and Vietnam and in part by Malaysia, Brunei and the Philippines.