BANGKOK (AFP) -- Thailand's police force said Friday it has launched an investigation into some 2,500 killings during this year's controversial "war on drugs" after the king called for an explanation of the death toll.

The police reacted speedily after the nation's highly respected King Bhumibol Adulyadej said in his birthday address Thursday that they should establish the circumstances and exact number of the deaths.

"National Police Chief General Sant Sarutanond has signed an order to set up a special police team to investigate all murder cases across the country from February 1 to April 30," said police Major General Pongsapat Pongcharoen.

Pongsapat said Sant had given the team seven days to report back to him and that all police agencies had been instructed to give their full cooperation.

The officers planned to begin meeting Monday and would reopen all the cases as well as look into the progress of cases where arrests had been made in connection with the deaths, he said.

Prime Minister Thaksin Shinawatra declared the war on drugs after the king voiced his grave concern over Thailand's narcotics addiction crisis in his birthday speech last year.

As the death toll mounted during the first three months of the no-holds-barred campaign, police argued most of the casualties were traffickers killed in turf wars, or from officers forced to act in self-defense.

However, Thai and foreign rights activists said that at least some of them were extra-judicial killings by security forces.

At a ceremony to declare victory in the war on drugs Wednesday, Thaksin dismissed the criticism and said he was not sorry to see "enemies of the nation" killed or sent to jail.

In his address to the nation the king praised the government's success in stamping out drugs but said that if it did not investigate the deaths it would continue to attract criticism.