Tensions in the disputed waterway have boiled over into violence in the past year, with a Filipino sailor losing a finger in the latest June 17 clash that Manila described as “intentional-high speed ramming” by the Chinese coast guard.

The US, a treaty ally, has offered support but Manila prefers to handle operations on its own, Armed Forces Chief General Romeo Brawner told Reuters late Thursday.

“Yes, of course, they have been offering help and they asked us how they could help us in any way,” he said.

“We try to exhaust all possible options that we have before we ask for help.”

Manila and Washington are bound by the 1951-Mutual Defense Treaty (MDT), a military pact that can be invoked in the case of armed attacks on Philippine forces, public vessels, or aircraft in the South China Sea.

Confrontations between the Philippines and China in Asia's most contested waters have increased in frequency over the past year as Beijing has pressed its claim to the waterway and Manila continued missions to bring supplies to soldiers living aboard a rusty, aging warship that it grounded on a contested shoal.

Some observers, including former deputy US National Security Adviser Matt Pottinger, have called for direct US naval support for the resupply missions.

But Philippine National Security Adviser Eduardo Ano said the Philippines wanted them to be a “pure Philippine operation”.

“This is our legitimate national interest, so we don’t see any reason for them (the US) to come in,” Ano told Reuters.

Ano, who spoke to his U.S counterpart Jake Sullivan last month to discuss shared concerns over China’s "dangerous and escalatory actions", said the MDT was “far from being invoked”.

"We (the Philippines and China) agreed that there will be some easing tension, but we will assert our rights, we will not compromise our national interest, and we will continue to fight and claim what is ours,' Ano said.

Neither official specified what support the US had offered.

Greg Poling, a South China Sea expert at Washington's Center for Strategic and International Studies think-tank, told Reuters he believed the US was open to naval escorts for the resupply missions to the stranded vessel. Washington has already provided some limited support, he said.

A Philippine official said last year Manila was consulting the US Army Corps of Engineers on how best to stabilise the BRP Sierra Madre, which was grounded on the contested Second Thomas Shoal, Poling said, while US aircraft have been filmed providing overwatch of the ship on multiple occasions.

The Permanent Court of Arbitration in the Hague ruled in 2016 that Beijing's expansive South China Sea claims via its nine-dash line had no basis under international law, but that has not stopped China, which rejects the ruling, from being more assertive in the waterway.

It has deployed coast guard vessels to patrol those areas, alarming the Philippines, rival Southeast Asian claimants and other states operating in the South China Sea, including the US, which is wary about China's growing military power and territorial ambition.

Military chief Brawner said the United States' offer of support, made in discussions at his level, was not a direct response to the June 17 incident but rather a reflection of the enduring military alliance between the two countries.

"It is really because of our being treaty allies, so that offer has been available to us for a long time not just because of the incident," Brawner said.

"But we did not ask them yet because as per the orders of our president we have to rely on ourself first."

The Pentagon did not immediately respond to a request for comment. Thursday was a federal holiday in Washington for the United States' Independence Day.

While China claims nearly all the South China Sea, a major shipping lane with about $3 trillion in trade passing through it annually, Vietnam, Taiwan, Malaysia and Brunei also claim parts.