The Philippines said Monday a former US naval base facing the South China Sea could play a key role as a hub for American ships as Washington moves to strengthen its presence in the Asia-Pacific.

Once the US military's largest overseas facility, the former Subic Bay naval base 80 kilometres (50 miles) northeast of Manila has been transformed into a freeport and tourism zone since it was shut down in 1992.

But a senior Philippine official pointed out that, with the United States planning to shift the bulk of its fleet to the Pacific by 2020 as it focuses on Asia, it would need natural deep water bays to dock its ships and submarines.

"Based on US official pronouncements, there is a strategic rebalancing (of its forces) and that means more assets, more aircraft in the Western Pacific," said Edilberto Adan, a former general who heads the government's Visiting Forces Agreement (VFA) commission.

"There are very few ports that can accommodate naval assets and naval carriers, and one of them is Subic.

"As the US begins to implement (the shift), Subic will play an important role because it is one of the important facilities that can service their presence in the Pacific."

He said Subic could "provide the necessary port calls, port visits and servicing required by US assets, naval or aircraft".

Adan was talking to reporters at Subic Bay aboard the USS Bonhomme Richard, an amphibious Marine Expeditionary Unit assault ship taking part in a 10-day joint exercises with Filipino forces.

Subic, along with the nearby Clark Airbase, were key facilities for the United States, the former colonial ruler of the Philippines, during World War II.

They then provided logistical support during the Vietnam War in the 1970s, and remained of strategic importance during the Cold War.

Clark closed down in 1991 after nearby Mount Pinatubo volcano erupted, covering the base in ash and making it unusable.

Subic, which is in the northern town of Olangapo facing the South China Sea, survived the eruption.

But, amid strong nationalist sentiment and street protests calling for US troops to leave the Philippines, the Senate voted in 1992 to end a lease agreement that allowed the bases to operate.

In November 1992 the last US ship sailed out of Subic.

The Philippines, however, ratified a visiting forces agreement with the United States in 1999, allowing the resumption of large-scale training exercises between the allies.

US troops have since been engaged in various exercises with the Philippines annually.

Adan, whose commission oversees the joint exercises with US troops, also said an increased American presence in the Philippines could help protect the surrounding seas.

"Our concern and everyone's concern in the region is freedom of navigation, to ensure that commerce and trade, commercial shipping go unhampered," he said.

"And the Philippines is in a very strategic location in the region, so it is important that (it) plays a role in that regional geographical configuration."

The Philippines has repeatedly expressed concern about a perceived more aggressive Chinese presence in the South China Sea.

China claims virtually all of the South China Sea, even waters close to the coasts of the Philippines and other Asian countries.

Tensions escalated this year after Philippine and Chinese ships were locked in a stand-off at a disputed shoal in the South China Sea.

The Philippines also accused China of using bullying diplomatic tactics to assert its claims, and sought expressions of support from the United States in its dispute.

Under a mutual defence treaty, the United States is bound to come to the defence of the Philippines if it is attacked.

However, Adan made no direct reference to China.