Manila said Wednesday the United States had a treaty obligation to help the Philippines if it is attacked on its own territory or in the South China Sea, as it rejected criticism of a security agreement.

President Barack Obama on Tuesday declared the US would support its ally in the event of being attacked, a day after his government signed an agreement allowing a greater American military presence on Philippine bases over 10 years.

Obama cited a 1951 mutual defence treaty but did not specifically mention coming to Manila's aid in the South China Sea, where China and the Philippines are in dispute over tiny islets, reefs and rocks.

"Under the mutual defence treaty, the United States 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," Foreign Secretary Albert del Rosario said in a statement.

Washington had affirmed in a 1999 diplomatic letter "that the South China Sea is considered as part of the Pacific area", del Rosario added.

Critics of the US security pacts as well as sections of the Philippine press allege Manila had gifted US troops with new bases but got nothing in return.

The Philippines has been embroiled in one of the highest-profile territorial disputes with China in the Spratlys, a chain believed to sit on huge oil and gas deposits, as well as another fish-rich South China sea atoll.

Manila alleges in a case filed in the United Nations last year that China claims 70 percent of the sea, even waters and islands or reefs close to its neighbours.

The Philippines, which has one of the weakest militaries in the region, has repeatedly called on longtime ally the United States for help as China has increased military and diplomatic pressure to take control of the contested areas.

It has described the new security pact as part of efforts to build up its military's deterrent capability to protect the country's exclusive rights under international law to sections of the sea closest to major Philippine islands.

US forces vacated major military bases in the Philippines in 1992 after the Philippine senate refused to ratify a new bases treaty.