By Manuel Mogato

MANILA (Reuters) - The Philippines accused China on Wednesday of reclaiming land on a reef in disputed islands in the South China Sea, apparently to build an airstrip, only a day after Washington described Beijing's actions in the region as "provocative".

If confirmed, the airstrip would be the first built by China on any of the eight reefs and islands it occupies in the Spratly Islands and would mark a significant escalation in tensions involving several nations in the area.

China claims almost the entire South China Sea, an area rich in energy deposits and an important passageway traversed each year by $5 trillion worth of ship-borne goods.

Brunei, Malaysia, the Philippines, Taiwan and Vietnam also have claims on the area.

Philippine Foreign ministry spokesman Charles Jose told Reuters that China had been moving earth and materials to Johnson South Reef, known by the Chinese as Chigua, in recent weeks. He said China was reclaiming land in violation of the Declaration on the Conduct of Parties in the South China Sea, an informal code of conduct for the region.

"They're about to build an airstrip," Jose said.

He said evidence of the Chinese activity on the reef had been shown in aerial photographs taken by the Philippine Navy. The Philippines and Taiwan already have airstrips in the area.

The ministry had already lodged a protest with the Chinese and raised the issue behind closed doors at last weekend's summit of the Association of South East Asian Nations in Myanmar, Jose said.

Tensions in the South China Sea were already high after China moved a large oil rig into an area also claimed by Vietnam. Beijing and Hanoi each accused the other of ramming its ships near the disputed Paracel Islands,

On Tuesday, Kerry said during a phone call with Chinese Foreign Minister Wang Yi that China's introduction of the oil rig and numerous government vessels into the area disputed with Vietnam was "provocative", a State Department spokeswoman confirmed.

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China in turn said there had indeed been provocative action taken in the area but that it was not the guilty party, with the foreign ministry blaming the United States for encouraging such behavior. The ministry said Wang had urged Kerry to "act and speak cautiously".

Beijing says the South China Sea issue should be resolved by direct talks between those involved and has bristled at what it sees as unwarranted U.S. interference.

It has also looked askance at the U.S. "pivot" back to Asia, especially Washington's efforts to boost existing military links with Tokyo and Manila.

The remote and otherwise unremarkable Johnson South Reef has been a catalyst for conflict in the past. In March 1988, China and Vietnam fought a brief naval skirmish on and around the reef with up to 90 Vietnamese reported killed.

(Writing by Paul Tait; Editing by Richard Pullin)