The Pentagon's No. 2 man has said Washington will not recognise an exclusion zone in the contentious South China Sea, if Beijing ever declares one.
China established in 2013 an air defence identification zone in the East China Sea, which covered islands also claimed by Japan, to strengthen its case of sovereignty over the disputed area.
Amid rising tensions in the South China Sea, United States Deputy Secretary of Defence Robert Work said yesterday that the US has told China it would not recognise a similar exclusion zone in the disputed waters, where countries in the region have overlapping territorial claims, according to Reuters.
China has dredged nearly 1,200ha of sand and coral to transform seven reefs in the Spratly archipelago west of the Philippines into islands with military-grade air strips and harbours, to press its claims to nearly all of the South China Sea.
These claims cover waters the Philippines insists are within its 200- nautical-mile exclusive economic zone. Vietnam, Malaysia, Brunei and Taiwan have their own claims.
Meanwhile, Manila has signed a US$114 million (S$154 million) contract to purchase two anti-submarine helicopters and may invest in its first submarine fleet, amid rising tensions in the disputed South China Sea.
Defence Undersecretary Fernando Manalo said the Philippines had agreed to buy two AgustaWestland AW159 Lynx Wildcat helicopters from Italian arms manufacturer Finmeccanica.
The helicopters, with a range of 490km, will be stationed aboard the frigates BRP Gregorio del Pilar and BRP Ramon Alcaraz, he said.
The weapons package may include British-made Sting Ray torpedoes, depth charges and anti-ship missiles.
President Benigno Aquino told a conference of newspaper publishers that the Philippines had been weighing having its own "submarine force" to meet changing security needs. "We might have to undergo changes to various aspects of our own military capabilities that have never been part of our needs."
The Philippines has been looking to place orders for three submarines since 2011. It has created a "submarine office", and it hopes to have a small fleet in operation in 10 years.
Defence analysts say having submarines will bring prestige to the navy, but it is not pragmatic, given the high costs and how little impact these will have operationally.
The Philippines has been ramping up defence spending in response to China's military expansion in the region. It has spent 41.2 billion pesos (S$1.2 billion) since 2012 to purchase, among other things, two US Coast Guard cutters and 12 FA-50 light fighter jets.