ASEAN Secretary General Dr Surin Pitsuwan noted that test-firing the rocket launcher could be perceived as a sign of regional instability. "We are very concerned with such development," he said, pledging a closer look at the situation.

Cambodia fired about 200 rounds from its Soviet-made BM21 rocket launcher in the mountains of remote Kampong Chhnang province within a range of 20 to 40 kilometres.

Cambodian Prime Minister Hun Sen reportedly said that the exercise was to prepare for the defence of his country and not about showing any military capabilities even though Cambodia and Thailand were at loggerheads.

Thai Prime Minister Abhisit Vejjajiva said yesterday that Thailand had no problem with the test, as it was a normal practice for a country to conduct military exercises.

Thailand and Cambodia have long outstanding border claims over an area surrounding the ancient Preah Viher Hindu temple, a UNESCO World Heritage site. The World Court ruled in 1962 that the temple belonged to Cambodia but it did not settle the issue of sovereignty over the disputed surrounding area, which has seen clashes between troops from the two sides.

When Cambodia named fugitive former Prime Minister Thaksin Shinawatra as its economic advisor late last year Thai-Cambodian relations took a nosedive. Mr Thaksin is on trial in Thailand where a court has recently ordered the confiscation of half of his wealth.

Carlyle Thayer, a military expert at Australia’s University of New South Wales, said the launch was “a bit of theatre” on Hun Sen’s part to maintain the support of the Cambodian military.