A senior US commander in the Pacific reaffirmed the United States' mutual defence treaty with the Philippines Sunday amid increased tensions between the archipelago and China.

In the strongest comments yet from an American official on the South China Sea dispute, Commander of the US Marines in the Pacific Lieutenant General Duane Thiessen said the Philippines and US were bound by a military agreement.

"The United States and the Philippines have a mutual defence treaty which guarantees that we get involved in each other's defense and that is self explanatory," he told reporters in Puerto Princesa, the capital of Palawan.

Thiessen was responding to a question about whether the US would provide assistance if Chinese armed forces attacked Philippine units over conflicting claims to the Scarborough Shoal which have flared up in recent weeks.

The US commander did not elaborate on what kind of assistance would be provided.

He also stressed that US-Philippine military exercises that began last week were not directed at China and not linked to territorial tensions.

"There is no direct linkage. There is no tie between Scarborough Shoal and US movement in the Pacific," he said.

His remarks came after Philippine Foreign Secretary Albert del Rosario issued a statement calling on other countries to take a stand on China's alleged aggressiveness in the region.

"Since the freedom of navigation and unimpeded commerce in the (South China Sea) are of great import to many nations, all should consider what China is endeavouring to do in the Scarborough Shoal," del Rosario said.

"All, not just the Philippines will be ultimately negatively affected if we do not take a stand," he said in an SMS message sent to reporters.

Chinese and Philippine ships began a standoff in Scarborough Shoal on April 8, giving increased attention to the annual joint military exercise this year.

A Philippine coast guard ship and two Chinese vessels are still facing off over the two countries' conflicting claims to the shoal, about 230 kilometres (140 miles) from the main Philippine island of Luzon.

A Chinese military paper recently warned that the latest US-Philippine exercise increased the threat of an armed confrontation in the region.

China claims all of the South China Sea, even up to the coasts of other countries including the Philippines, directly conflicting with the claims of many of its neighbours.

While the Philippines concedes it cannot stand up to China's military might, officials have also cited its 1951 mutual defence treaty which calls on the United States to come to the country's aid in case of external attack.