nhk world's charmaine deogracias reports. >> reporter: members of the u.s. forces combined for a joint exercise known as valiant shield. they conduct the drill every two years to prepare for an emergency in asia. we're aboard "u.s.s. george washington" in the pacific ocean. it's just one of two aircraft carriers participating in exercise valiant shield. with 18,000 troops jointly training for air-to-sea combat. military officials expanded the exercise this year so that it was more than twice as big as last time. they included two aircraft carriers and more than 200 fighter jets. the officials are responding to china's increasingly assertive maritime activities. beijing has been expanding its reclamation work on a reef among the spratly islands in the south china sea. the philippines and other countries have territorial claims there. officials in the philippines, a longtime ally of the u.s., say china has started construction of what appeared to be military installations. chinese military officials are also stepping up their anti-access area denial strategy.