CHINA has almost finished building nearly two dozen constructions on its artificial islands in the South China Sea, intelligence officials have said.

The US officials suggested the structures are designed to house long-range surface-to-air missiles, as tensions continue to boil over the disputed islands.

Speaking to Reuters on condition of anonymity, they said the constructions could constitute a “military escalation”.

“It is not like the Chinese to build anything in the South China Sea just to build it, and these structures resemble others that house SAM batteries, so the logical conclusion is that’s what they are for,” one of the US officials said.

Another official said the structures appeared to be 20 meters long and 10 meters high.

The official said the developments did not pose a significant military threat to US forces in the region, given their visibility and vulnerability.

He said it was likely more a political test to see how the Trump administration might respond.

In recent months, President Trump’s administration has taken a tougher stance on the South China Sea, but their activity in the region remains difficult to predict.

Beijing has acknowledged the report, but would not confirm whether it intended to place missiles on the reefs.

“China carrying out normal construction activities on its own territory, including deploying necessary and appropriate territorial defence facilities, is a normal right under international law for sovereign nations,” China Foreign Ministry spokesman Geng Shuang told reporters.

A Pentagon spokesman said the United States remained committed to “non-militarisation in the South China Sea” and urged all claimants to take actions consistent with international law.

US officials also said they are seeing signs Russia is preparing to deliver advanced SA-21 launchers to China, which could eventually end up on the islands, Fox News reports.

Other world leaders in the region have expressed concern over the findings.

At a retreat on the Philippine island of Boracay, the Association of Southeast Asian Nations (ASEAN) met and stressed that demilitarisation of all parties was crucial.

“The ASEAN ministers have been unanimous in their expression of concern about what they see as the militarisation of the region,” Philippine Foreign Secretary Perfecto Yasay told reporters.

“In so far as certain reclamation of certain features built on the South China Sea that had been completed, they (ASEAN ministers) have noticed, very unsettlingly, that China has installed weapons systems in these facilities that they have established.”

Vietnam, Malaysia, the Philippines and Taiwan also make claims to the South China Sea.

Earlier this week, the US deployed the aircraft carrier USS Carl Vinson to the disputed waters, saying it was part of maritime “routine operations”.

The Vinson carries a flight group of more than 60 aircraft, including F/A-18 jet fighters.

China’s Foreign Ministry responded unfavourably to the deployment days before it happened, warning Washington against challenging its sovereignty.

“China respects and upholds the freedom of navigation and overflight in the South China Sea, which countries enjoy under international law, but firmly opposes any country’s attempt to undermine China’s sovereignty and security in the name of the freedom of navigation and overflight,” Mr Shuang said.

The latest findings tie into a warning Sir Angus Houston made earlier this month.

Speaking at a conference in Canberra, the former head of the Australian Defence Force warned that China’s presence in the disputed region was permanent.

“In my view it is too late to stop the China program in the South China Sea,” said Sir Angus at a conference in Canberra. “What is important now is to ensure freedom of navigation and the right of innocent passage.

“I have seen the imagery (and) what you see is infrastructure going in, and it is not going to be too much longer before it is fully developed.

“All of this development will enable China to dominate the South China Sea and extend its permanent military presence further south in proximity to Indonesia, Malaysia and Singapore.”