Mahn Sha Lar Phan, secretary-general of the Karen National Union (KNU), was shot at his two-storey wooden home by two men who arrived in a pickup truck, his wife Kim Suay told Reuters at the scene. He died instantly.

"One of them walked up to the house and said in Karen 'How are you, uncle?' Then the other man joined him after parking the truck and they both shot him with two pistols," she said, her voice shaking with emotion.

In an interview with Reuters on Monday, he had predicted a possible increase in violence ahead of a constitutional referendum in the former Burma in May.

However, the KNU and its armed wing, the Karen National Liberation Army (KNLA), are riven by internal feuds and lethal vendettas.

His son Hse Hse, another senior member of the predominantly Christian Karen rebel movement, blamed a Buddhist Karen splinter group which brokered a truce with the Myanmar junta in the mid-1990s.

"This is the work of the DKBA and the Burmese soldiers," Hse Hse said, refering to the Democratic Karen Buddhist Army.

The Karen have been fighting for independence in the hills of eastern Myanmar for the last 60 years, one of the world's longest-running insurgencies.

Thai police said they had the registration number of the truck and were setting up roadblocks around Mae Sot, a "wild west" frontier town of refugees, illegal migrants and gem dealers, to try to catch the two killers.