Malaysia has released 6,000 genetically modified mosquitoes designed to combat dengue fever, in a landmark trial slammed Wednesday by environmentalists who say the experiment is unsafe.

In the first experiment of its kind in Asia, about 6,000 male Aedes aegypti mosquitoes were released at an uninhabited site in the central state of Pahang, the government-run Institute of Medical Research (IMR) said.

The IMR, which was tasked with carrying out the trial, said the experiment was conducted on December 21 to "study the dispersal and longevity of these mosquitoes in the field".

"The experiment was successfully concluded on January 5, 2011," the institute said in a statement dated Tuesday, adding that no further releases are planned until the trial results are analysed.

The insects in the experiment have been engineered so that their offspring quickly die, curbing the growth of the population in a technique researchers hope could eventually eradicate the dengue mosquito altogether.

Females of the Aedes species are responsible for spreading dengue, a deadly disease which killed at least 134 people last year in Malaysia alone.

The trial has sparked widespread concern among environmental groups and non-government organisations (NGOs), and had been postponed due to their protests as well as unfavourable weather conditions.

"I am surprised that they did this without prior announcement given the high level of concerns raised not just from the NGOs but also scientists and the local residents," said researcher Lim Li Ching from Third World Network.

The network is part of 29 public health and environmental groups which have repeatedly demanded the government cancel the trial, saying it was risky and could lead to unintended consequences.

"We don't agree with this trial that has been conducted in such an untransparent way. There are many questions and not enough research has been done on the full consequences of this experiment," she told AFP.

Critics have also said that too little is known about the Aedes mosquito, and how the genetically modified insects would interact with their cousins in the wild.