The US Pacific commander Tuesday emphasised the Philippines' importance as a military ally, as Filipino forces were involved in an increasingly tense standoff with Chinese ships in the South China Sea.

"Our 62-year alliance with the Philippines remains key to our efforts to ensure the stability and prosperity of the Western Pacific," Rear Admiral Robert Thomas told reporters in Manila.

He sailed to the Philippine capital on Tuesday aboard his command ship, the USS Blue Ridge, days after the latest of a series of hostile encounters between the Philippines and China in the South China Sea.

China said its coastguard on March 9 blocked two Philippine-flagged vessels approaching Second Thomas Shoal, which is guarded by a small group of Filipino marines but is also claimed by China.

The Philippine military evaded the blockade by airdropping supplies to the troops.

The shoal is part of the Spratlys, a chain of islets and reefs that sit near key shipping lanes, are surrounded by rich fishing grounds and are also believed to lie atop huge oil and gas reserves.

They are around 200 kilometres (125 miles) from the western Philippine island of Palawan and about 1,100 kilometres from the nearest major Chinese land mass.

China claims most of the South China Sea, even waters approaching the coasts of its neighbours.

The Philippines grounded an old navy ship at Second Thomas Shoal in 1999, four years after China built structures on a nearby Filipino-claimed reef. Filipino troops have kept a presence on the ship ever since.

On Monday, Chinese foreign ministry spokesman Hong Lei warned that China "will never allow any form of occupation" of the Second Thomas Shoal.

He repeated an allegation, denied by Manila, that the Philippine vessels were bringing construction materials there on March 9.

"China watches closely and is highly vigilant on further possible provocations in the South China Sea by the Philippines and it must bear all the consequences arising therefrom," Hong said.

Asked what the US 7th Fleet would do to help the Filipino marines, Thomas said he did not wish to address "hypothetical" scenarios, but then highlighted his country's 1951 mutual defence pact with Manila.

"And so without going into the hypotheticals, what I would offer is that the 7th Fleet is going to support this alliance. Period," he said.

The pact binds each country to come to the other's aid if its armed forces or ships are attacked in the Pacific.