Manila expects China to try to build on a reef off the coast of the Philippines, the country's defence secretary said Tuesday, adding this would be "unacceptable" in the flashpoint waterway.

In an interview with AFP, Delfin Lorenzana said he believed China would eventually reclaim Scarborough Shoal, just 230 kilometres (143 miles) from the main Philippine island of Luzon.

Beijing has already built up a number of islets and reefs in the Spratly Islands in the South China Sea, installing military facilities on several of them.

Analysts say similar installations on nearby Scarborough Shoal could give China effective military control over the disputed Sea -- something the US has said it is not prepared to accept.

"They encroached," Lorenzana said of a 2012 confrontation that saw Philippine vessels displaced from the shoal. "They occupied three islands there (in the Spratlys) plus they are trying to get Scarborough. So to us that is unacceptable".

"If we allow them, they will build. That's very, very disturbing. Very much (more) disturbing than Fiery Cross because this is so close to us," Lorenzana added, referring to one of the Philippine-claimed reefs China has built on.

Because of its position, another military outpost at Scarborough Shoal is seen as the last major physical step required to secure control of the sea.

An outpost there would also put Chinese fighter jets and missiles within easy striking distance of US forces stationed in the Philippines.

The shoal also commands the northeast exit of the sea, so a Chinese military outpost there could stop other countries' navies from using the waterway.

A UN-backed tribunal -- in a case brought by Manila under then-president Benigno Aquino -- ruled last year that the so-called "nine-dash-line" which underpins Beijing's claim to most of the South China Sea had no legal basis.

But his successor Rodrigo Duterte has courted China and backed away from his country's close relationship with the United States.

Lorenzana said the Chinese island reclamation was intended to secure control of the South China Sea.

"That could be their strategy to counter any superpower that would encroach on South China Sea because they believe South China Sea is -- that's like their lake to them -- theirs," he added.

- 'Red line' -

The administration of new US President Donald Trump has indicated it will push back against any Chinese attempt to bolster control of the sea.

During confirmation hearings, Secretary of State Rex Tillerson said the US would block Chinese access to reclaimed islands, although analysts have pointed out that this would require a military blockade -- an act of war.

Lorenzana called Tillerson's remark "very troubling", adding the Philippines would become the battleground if conflict broke out between the two superpowers.

He added that China tried to build on Scarborough last year but American warnings stopped them.

"The Americans, that's their red line. Red line meaning you can't do that there, so they (China) did not do it."

"If we had a strong military presence (in the South China Sea), we can stop them (China) but we don't. I am still hoping in the future some reasonable guy there in Beijing will come to see the light that this is ours."

"That is shooting for the moon but who knows?" Lorenzana added.

He said the Philippines would soon repair a runway on Thitu island, one of its garrisoned features in the Spratlys which he planned to visit, and put up "additional barracks for the marines there".

The defence chief said Manila would try to "manage" the maritime dispute while working with Beijing in other areas like patrolling piracy-plagued southern Philippine waters.