Human rights groups in Thailand are expressing concern over the murders of environment and human rights defenders in the country.

Amnesty International says it is gravely concerned by the killing of Charoeun Wataksorn. He was shot to death hours after he testified at a hearing on alleged land grabbing in his home province of Prachuab Khiri Khan, 230 kilometers southwest of Bangkok.

The Thailand director of the human rights group, Srirak Plipat, says the Thai government is not protecting people like Mr. Charoeun.

"We call on the Thai government to initiate independent and effective investigations on the [Charoeun] case and we call on the government to provide protection to other human rights defenders," he said.

Thai Prime Minister Thaksin Shinawatra has ordered police to quickly hunt down the murderers and unmask the mastermind behind the crime.

Mr. Charoeun successfully led a 10-year campaign against the construction of a coal fired electrical generating plant in his region. The government eventually scrapped the plan and proposed a smaller plant fired by natural gas.

More than a dozen activists have been murdered in the past three years. But Amnesty International's Mr. Srirak says, that according to information given to his group, only one case has been taken to court.

"The government has to step up and defend human rights defenders," he said. "There is still too little being done to protect them. And the situation has been getting worse."

He says activists working in remote rural areas are the most vulnerable, adding that in the past six months, six community leaders who led protests against construction and illegal logging projects have been killed.