US Secretary of Defense Ashton Carter says its forces will gain access to more military bases in the Philippines, on top of the current five. Carter on Wednesday began a visit to the long-standing Asian ally. Earlier this year, Manila announced it would allow the US to use five of its installations, including an air base close to the South China Sea.

The agreement that went into force in January aims to strengthen the Philippines' defensive capabilities amid a tense maritime dispute with China. At the same time, it will help the Pentagon pivot more of its forces toward Asia.

Meanwhile, defense officials from the Philippines and Vietnam will meet this week to explore possible joint exercises and navy patrols. But officials have said a deal this week was unlikely.