JAKARTA (Reuters) - Four Indonesian soldiers were killed and 8 wounded on Wednesday after they were hit by artillery fire during a rapid response training exercise in the Natuna islands north of Borneo in the South China Sea, a military spokesman said.

“The army leadership has conveyed its deepest condolences after the death of four excellent soldiers in an accidental incident in Natuna,” said spokesman Denny Tuejeh.

The Indonesian army had launched an investigation to examine how the anti-aircraft weapons system had fired upon the group of soldiers during the exercise.

The wounded personnel had been evacuated and were being treated at a nearby hospital, he said.

Indonesia has been holding regular exercises and has increased its military presence on and around the islands that are located near disputed waters in the South China Sea.

While the sovereignty of the islands is not disputed, Jakarta has objected to Beijing’s inclusion of waters around them within China’s “nine-dash line”, a demarcation line used by Beijing to show its claims.