The Philippines and the United States have started 12 days of joint military manoeuvres, which China has criticised as "fear-mongering and untimely".
About 8,000 troops from both sides are taking part in the Balikatan (shoulder-to-shoulder) exercises being held across the Philippines, with at least one drill scheduled to take place in waters off Palawan province, just outside the disputed South China Sea.
About 80 soldiers from the Australian Defence Force are also joining the exercises.
Japan, which has asked to be included as a regular participant, and Vietnam have sent observers.
Singapore, which participated as an observer in the past, is not participating this year, and was also not part of last year's exercise.
Vice-Admiral Alexander Lopez, exercise director for the Philippine side, said the drills were "designed not to address a particular concern, but the whole lump in the spectrum of warfare".
"China is not part of the idea," he stressed.
Yesterday, the official Xinhua news agency slammed the Philippines in a commentary headlined "Manila's dangerous, futile game of involving outsiders in regional row". It said the exercise "caps Manila's recent attempts to involve outsiders in a regional row".
"However, a provocation so fear- mongering and untimely as such is likely to boomerang on the initiators," it added.
China's claim to nearly all of the South China Sea as well as its reclamation and construction activities on disputed features in the waters have alarmed other countries, such as the Philippines and Vietnam, which have overlapping claims.
Beijing views Washington's involvement in military exercises with American allies in the region as part of the United States' policy of containment.
"That's not part of our objective," Lieutenant-General John Toolan, commander of US Marine forces in the Pacific, told reporters yesterday. "Our objective," he said, "is to make sure that we can work together and have a responsive crisis capability."
Asked whether such a potential crisis involves the South China Sea, he said: "It does, absolutely."
Underscoring the military exercises' significance, US Defence Secretary Ash Carter will fly to the Philippines to observe live-firing drills.
He will be the first US defence secretary to take part in Balikatan.
Lt-Gen Toolan said Mr Carter is taking a specific interest in a mobile multiple rocket system that will be used for the first time in the exercise.
The missile platform - known as the high mobility artillery rocket system, or Himars - consists of six rockets mounted on a truck frame.
Major-General Rodolfo Santiago, assistant exercise director, said Himars was included in the exercise so that the Philippines can get a closer look at a missile system that is affordable and can be rapidly deployed to better secure its borders.
He said Himars can even be deployed on one of the Philippine- held islands in the South China Sea.
This year's Balikatan will also see Australian troops joining their US and Philippine counterparts in a simulation drill to retake an oil platform located off Palawan, the province closest to the Spratly island chain, where China has created artificial islands to bolster its territorial claims.
Correction note: An earlier version of this story stated that Singapore was an observer in last year's Balikatan exercises. Mindef has clarified that Singapore did not take part last year.