WASHINGTON (AP) — The Philippines' top diplomat said Tuesday he is seeking more U.S. help in stopping massive land reclamation by China that could give Beijing effective control of the South China Sea.

Foreign Minister Albert del Rosario told a Washington think tank that China is attempting to enforce its so-called "nine-dash line" — the rough demarcation of China's territory on its official maps that virtually envelopes that ocean. He described the line as "unlawful."

China and five other claimants, including the Philippines, contest control of the potentially resource-rich waters.

The United States, a treaty ally of the Phillipines, has expressed growing concern over China's creation in the past year of artificial islands, particularly in the Spratly Islands chain. A senior U.S. defense official said last week that China's land reclamation, potentially for military use or airstrips, now totals about 2,000 acres.

"We are taking the position that we must do something quickly lest the massive reclamation results in de facto control of China of the South China Sea," del Rosario told the Center for Strategic and International Studies in Washington.

He said that China's control could lead to militarization, and also threatens the rule of law and freedom of navigation.

Chinese officials defend the reclamation, saying it is Beijing's territory and that the buildings and infrastructure are for public service use and to support fishermen. It accuses the Philippines, Vietnam and others of carrying out their own building work on other islands.

The U.S. says it takes no position on sovereignty claims in the South China Sea, but has an interest in peace and stability in its busy shipping lanes. It is providing millions in aid to boost the Philippines' capability in maritime security and has voiced support for Manila's ongoing legal challenge to China's nine-dash line.

But del Rosario said the U.S. can do more boost its engagement in Asia, including stronger economic ties.

"We are trying to deliver the message that the Asia pivot is not as focused and as strong as it should be," he said.

Del Rosario was speaking after Senate Democrats blocked efforts to begin a full-blown debate on a trade bill that is key to President Barack Obama's ambition to complete a pan-Pacific free-trade agreement. The Philippines is not among the 12 nations negotiating the agreement, but del Rosario said Manila hopes to join eventually.