FORT MAGSAYSAY, March 13 (Reuters) - The Philippines and United States launched army-to-army exercises on Monday, with a focus on enhancing the Southeast Asian nation's ability to protect and defend its territory from external threats.

The drills come on the heels of President Ferdinand Marcos Jr's decision last month to expand the United States' access to his country's military bases - a move that has infuriated China since the Philippines is seen as a fulcrum of geopolitical rivalry between the two major powers.

More than 3,000 Filipino and U.S. soldiers will participate in the three-week long annual exercises called Salaknib, which involve multiple small-arms live-fire exercises, artillery and mortar live-fire events, and construction projects.

"The scenarios would involve the defence of the Philippine archipelago from potential foreign aggressors," Philippines Army Chief Lieutenant General Romeo Brawner told reporters following the opening ceremony.

"Since this is an army-to-army exercise, we will focus on defence operations such as air defence and also our defence from the shorelines," he added.

The majority of activities will take place at Fort Magsaysay, the Philippines' largest military camp, and one of the five existing sites the United States has access to under its Enhanced Defence Cooperation Agreement (EDCA) with Manila. Under the agreement, the United States can use the bases for joint training, pre-positioning of equipment and building of facilities such as runways, fuel storage and military housing, but not to maintain a permanent presence.

China has slammed the expanded agreement, calling it "part of U.S. efforts to encircle and contain China through its military alliance with this country".

"By doing these, the U.S. has not only heightened tension, driven a wedge between China and the Philippines, but also has disturbed and upset the joint effort of countries in this region to safeguard peace and stability in the South China Sea," the spokesperson of the Chinese Embassy in Manila said in a statement on Sunday.

The Philippines has yet to disclose the additional bases to which the United States will have access, but a former military chief has said they include bases on the island of Luzon, facing north towards Taiwan, and on Palawan in the southwest, near the disputed Spratly Islands in the South China Sea.