MANILA (Reuters) - The United States has a treaty obligation to help the Philippines in case of an attack on its territory or armed forces in the South China Sea, the Philippine foreign minister said on Wednesday, rejecting questioning of a security pact.

The United States and the Philippines on Monday signed a 10-year Enhanced Defense Cooperation Agreement allowing U.S. forces wider access to Philippine bases and to position ships, aircraft, equipment and troops for maritime security.

The deal was testimony to an "ironclad" U.S. commitment to defend its oldest Southeast Asian ally, U.S. President Barack Obama told troops from both countries this week while on a visit to Manila.

But Obama did not say categorically the United States would defend the Philippines in its territorial disputes with China in the South China Sea, leading to some questions in the Philippines about the scope of the pact.

Philippine Foreign Secretary Albert del Rosario issued a statement on Wednesday apparently aimed at dispelling doubts about the deal.

"Under the Mutual Defense Treaty, the U.S. will come to the assistance of the Philippines if our metropolitan territory is attacked or if our armed forces are attacked in the Pacific area," del Rosario said.

"In 1999, in a diplomatic letter, the U.S. affirmed that the South China Sea is considered as part of the Pacific area."

China claims virtually the whole of the South China Sea and has in the past couple of years been much more assertive in staking its claim.

In 2012, China seized control of a disputed reef known as the Scarborough Shoal, preventing Filipino fishermen from getting near the rocky outcrop.

Last month, the Chinese coastguard blocked two civilian Philippine ships from delivering food and water to soldiers deployed aboard a Philippines navy transport vessel at another disputed spot, Second Thomas Shoal.

They continue to blockade the area.

HARD WORK

The Philippines in March lodged a case against China at an arbitration tribunal in The Hague to try and settle the festering territorial dispute.

Story continues

Brunei, Malaysia, Vietnam and Taiwan also have claims on the South China Sea, where $5 trillion of ship-borne trade passes every year and is believed to have rich deposits of oil and gas.

China's Foreign Ministry said maintaining peace and stability "accords with all sides' interest" and would require hard work by all sides.

"The (Chinese) people have reason to demand that any agreement signed by the United States and the Philippines accords with this principle and does not harm mutual trust with other countries in this region and regional peace and stability," ministry spokesman Qin Gang told reporters at a regular briefing.

Some Philippine legislators and non-government organizations have criticized the security deal with the United States as unconstitutional and said it unfairly favors the United States.

"Through the Enhanced Defense Cooperation Agreement, the Philippine government has surrendered our sovereignty to free-loading U.S. forces," said Renato Reyes of the left-wing Bayan (Nation) group.

(Reporting By Manuel Mogato, Additional reporting by Ben Blanchard in BEIJING; Editing by Michael Perry and Robert Birsel)