SAN ANTONIO, Philippines - Scores of US and Filipino marines launched mock assaults on a South China Sea beach in the Philippines Friday, as a row escalates between Beijing and its smaller neighbours over the strategic waters.
The drills, part of annual war games between the two countries, came after Vietnam said its vessels were rammed by Chinese ships protecting a deep-sea oil rig in disputed waters off Vietnam's coast.
The flare-up - which saw counter-accusations from Beijing which said its own vessels had been repeatedly attacked - sparked concern from Japan, the United States, and the European Union about deteriorating security in the region.
The US-Philippine exercises - involving about 5,500 troops - included Friday's drills on a beach about 220 kilometres (135 miles) from Scarborough Shoal, which China took control of two years ago after a stand-off with the much-weaker Philippine forces.
Three US rubber raiding craft and two small-unit Filipino riverine boats practiced stealth landings from before dawn at the desolate beach inside a northern Philippines navy base.
The teams of about 40 US and 80 Filipino marines scrambled up the sloping shore with assault rifles to surround a mocked-up enemy tent before running back to their boats.
Two Filipino navy ships served as launch pads for the amphibious units.
"There was no specific scenario," US Marines spokesman Captain Jeremy Scheier said when asked if they had an enemy target in mind for the drills.
Filipino Foreign Secretary Albert del Rosario said last week that the joint manoeuvres were designed to help the hosts boost their "maritime capability" to address "challenges" in the South China Sea.
China claims sovereignty over almost the whole of the South China Sea, which is also claimed in part by the Philippines, Vietnam, Taiwan, Brunei, and Malaysia, and is believed to sit atop vast oil and gas deposits.
The latest flashpoint has been China's unilateral decision to move the rig into waters off Vietnam, triggering a serious confrontation between the two countries which fought a brief border war in 1979.
"We can expect a few more months of high tension between the two countries, and things could get out of hand and shots could be exchanged," said Ian Storey, a security expert at Singapore's Institute of Southeast Asian Studies.
Neither country was expected to back down, he said, though Vietnam was expected to be "on its own" in the confrontation.
"In incidents like this, the US has very limited options on how to respond," he added.
Storey said the fracas was part of a "pattern of behaviour we have seen over the last few years in which China has been trying to enforce its jurisdictional claims" over large chunks of the South China Sea.
The annual war games began on May 5, shortly after US President Barack Obama's tour of Asian allies - Japan, South Korea, Malaysia and the Philippines.
When in Manila, he warned China against using force in territorial disputes, and said Washington would support the Philippines in the event of an attack.
While the United States has said its defence commitments with Japan include islands in the East China Sea administered by Tokyo but claimed by China, it takes no position in the other territorial disputes involving its Asian allies.
The Philippines asked a United Nations tribunal in March to declare China's claims over most of the sea as illegal. Beijing has rejected arbitration.
On Tuesday, Filipino police detained a Chinese-flagged fishing vessel and its 11 crew members near Half Moon Shoal, 106 kilometres west of the large western Philippine island of Palawan, which is claimed by both countries.
Manila has ignored a Chinese demand to free the vessel and crew.