The two battling ethnic Karen groups have been fighting on and off for 12 years since the Democratic Karen Buddhist Organisation (DKBO) split from the Karen National Union (KNU).
The KNU is the largest and oldest Burmese insurgent group.
The two groups have had differences over religion, and the DKBO joined forces with the Burmese government two years ago.
Most members of the KNU are Christian. Separately, the official Thai News Agency reports a clash between Thai Army Rangers and an unknown armed group in the northern Mae Hong Son province.
One Thai soldier was killed in the Wednesday night skirmish.
