US Defense Secretary Ash Carter will observe the drill after arriving next week, the first US defense chief to do so, Reuters said.

Manila played down any suggestion that the Balikatan ("Shoulder-to-Shoulder") exercise, which will simulate retaking an oil-and-gas platform and practice an amphibious landing, targeted any specific country.

Asked if the drill's scenarios include a potential security crisis in the South China Sea, Lieutenant-General John Toolan, commander of US Marine forces in the Pacific, said, "It does, absolutely," according to Reuters.

Toolan said a mobile rocket system that has been deployed in various areas, including Afghanistan, will be used during the exercise for the first time. "We can move this stuff anywhere we need to."

Rene de Castro, an international studies professor at De La Salle University in Manila, told Agence France-Presse that the drills appeared to have China's presence in the South China Sea in mind.

"Looking at the features of Balikatan — the mobile missile-launchers, the fighter planes — that is an indication that the alliance is being geared for territorial defense," he said.

President Xi Jinping told US President Barack Obama on March 31 during their meeting on the sidelines of the nuclear summit in Washington that China would not accept any behavior under the guise of freedom of navigation that violates its sovereignty.

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