The Philippines accused China on Wednesday of reclaiming land on a reef in the disputed Spratly Islands in the South China Sea, apparently to build an airstrip, only a day after Washington described Beijing’s actions in the area 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 group 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 one of the world's busiest sea lanes.

Philippine Foreign Ministry spokesman Charles Jose told Reuters that China had been moving earth and materials to Johnson Reef in recent weeks. He said China was reclaiming land in violation of the Declaration of 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. However, Foreign Secretary Albert del Rosario told The Associated Press, "We're not exactly sure what are their intentions there."

Del Rosario said the Philippines had lodged a protest against China last month after surveillance aircraft confirmed, and took pictures of, the reclamation and dredging being done by Chinese vessels at the Johnson Reef.

China replied to the Philippine protest by saying it owned the reef, he said.

Another senior government official, who spoke on condition of anonymity because he was not authorized to talk about the issue, said China's reclamation was first detected by air force planes six months ago. Philippine aircraft searching for the missing Malaysian jetliner in March also spotted the continuing reclamation on the submerged Johnson Reef by at least one Chinese ship, backed by smaller vessels.

Manila estimates that the reclamation has turned the submerged reef and a sandbar into a 74-acre land mass that transformed an underwater outcropping into an islet, a senior diplomat told The AP on condition of anonymity because of a lack of authority to discuss the issue.

It's the latest territorial flare-up between neighbors in the potentially oil- and gas-rich region. Tensions in the South China Sea were already high after China moved a 40-story-tall, billion-dollar oil rig into an area also claimed by Vietnam.

On Tuesday, U.S. Secretary of State John 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."

China 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 U.S. for encouraging such behavior. The ministry said Wang had urged Kerry to "act and speak cautiously."

Feelings spilled over onto land as anti-China mobs torched up to 15 foreign-owned factories and trashed many more in southern Vietnam. The unrest late Tuesday at a Singapore-run industrial park and others nearby followed protests by up to 20,000 workers at the complexes in Binh Duong province. Smaller groups attacked factories they believed were Chinese-run, but some were Taiwanese or South Korean, the VnExpress website quoted Tran Van Nam, the deputy head of the province's people's committee, as saying.

Beijing wants the dispute to 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" to Asia, especially Washington's efforts to boost existing military links to Tokyo and Manila.

Del Rosario said the Philippines raised the reclamation issue, along with the deployment of Chinese coast guard ships at the Second Thomas Shoal and "harassments of our fishermen," during a summit of the Association of Southeast Asian Nations (ASEAN) last week in Myanmar. Four members of the 10-nation bloc — Brunei, Malaysia, the Philippines and Vietnam — are engaged in territorial disputes in the Spratlys with China and Taiwan.

ASEAN issued a statement expressing concern over recent territorial spats in the South China Sea after the summit, attended by Southeast Asian heads of state.

Al Jazeera and wire services