A Burmese rebel leader who was an outspoken critic of the country's military government has been shot and killed at his home near the border with Thailand.

Karen National Union Secretary General Pado Mahn Sha Lah Phan was killed Thursday at his home in the Thai border town of Mae Sot.

At least two unidentified men were seen carrying out the killing, but no one has claimed responsibility.

Police and family members suggest that rebel splinter groups were involved.

In an interview with Reuters, Man Sha's son blamed the Democratic Karen Buddhist Army for the attack. The predominantly Christian Karen National Union frequently clashes with the Democratic Karen Buddhist Army, which has links to the Burmese military.

Other reports suggest that another breakaway Karen faction, the 7th Brigade group, may have been involved.

The shooting incident comes at a politically sensitive time, less than a week after Burma's military leaders announced plans to hold a referendum in May and general elections in 2010.

In interviews earlier this week, Mahn Sha predicted there would be an increase in violence in the lead-up to the government's planned referendum on a new constitution. He denounced the referendum as a fraud and an attempt to hold onto power.

Mahn Sha also was a top leader of the National Council of the Union of Burma, a Thai-based group that opposes military rule in Burma.

The Karen National Union has been fighting for autonomy in eastern Burma for decades. It is the only major ethnic rebel group to not sign a cease-fire agreement with the Burmese government.

Some information for this report was provided by AP and Reuters.