Officials say Vietnam has moved mobile rocket launchers to several islands in the South China Sea. The move is likely to raise tensions with China in the disputed area.

Western diplomats and military officials told the Reuters news agency that Vietnam moved the launchers to five bases in the Spratly Islands in recent months. They say these rocket launchers can attack Chinese military locations in the disputed waters.

Reuters reports that the launchers cannot be seen from the air and have not yet been armed. But sources say they could be armed with rockets within two or three days.

Vietnam’s foreign ministry said the information was “inaccurate” without giving any detail.

In June, Vietnam's Deputy Defense Minister said the country had no such launchers or weapons in the Spratly Islands. But he added that Vietnam had the right “to self-defense to move any of our weapons to any area at any time within our sovereign territory."

On Wednesday, China’s Foreign Ministry released a statement saying "China has indisputable sovereignty over the Spratly Islands and nearby waters." The statement also says that China opposes “illegal construction and military deployments” on the islands.

Carl Thayer is a Vietnam expert at the Australian Defense Force Academy. He said Vietnam’s defensive movements show how serious the country is about possible hostile actions by China.

He added that “China is unlikely to see this as purely defensive, and it could mark a new stage of militarization of the Spratlys."

In 1988, China and Vietnam fought a sea battle to control parts of the Spratly Islands. Sixty-four Vietnamese soldiers were killed trying to defend the country’s flag on South Johnson reef.

Last February, Vietnam condemned China’s deployment of missiles on Woody Island in the South China Sea. Vietnam said it "threatens peace and stability in the region as well as security, safety and freedom of navigation and flight."

Military experts believe the rocket launchers are part of a rocket system purchased from Israel. The system, known as EXTRA, can attack both ships and land targets up to a range of 150 kilometers. EXTRA is considered to be easily movable and effective against multiple targets at the same time.

Experts say it is the most noteworthy defensive move Vietnam has made in the South China Sea in more than 20 years. It is designed to counter China's build-up on reclaimed islands in the disputed waters.

Vietnam placed the launchers after an international court ruled against China in a case brought by the Philippines.

The July ruling dismissed China’s claim of “historic title” to much of the South China Sea.

I’m Jonathan Evans.