MANILA - Manila expects China to try to build on a reef off the Philippines' coast, the country's defense secretary said Tuesday, a move he said would be "unacceptable" in the flashpoint waterway.

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

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

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

"They encroached," Lorenzana said of a 2012 confrontation that saw Philippine vessels displaced. "They occupied three islands there 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 at the shoal 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 vital stretch of waters.

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 Beijing and backed away from his country's close relationship with the United States.

Lorenzana said Chinese island-reclamation efforts were meant to control 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.

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

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