One of the few ethnic armies not to have agreed a truce with the Burmese government has claimed responsibility for the killing of 20 troops last week.

The Shan State Army-South (SSA-S) said in a statement that it had launched an ambush against a joint column of Burmese infantry in Namhsan town, near to Lashio in Burma’s northeastern Shan state.

Twenty soldiers, including one captain, were killed in a fight which lasted three hours, the statement said. No troops belonging to the Shan army, one of the bigger rebel factions fighting the government, were reported killed.

The Burmese army has intensified its presence in Shan state, where it awaits a decision over whether the country’s largest ceasefire group, the 30,000-strong United Wa State Army (UWSA), will transform into the contentious Border Guard Force.

Pressure from the ruling junta on all of Burma’s 18 ceasefire groups to transform and come under the direct control of Naypyidaw has ratcheted up tension in the already volatile ethnic states.

Observers have warned of the potential for widespread fighting unless the junta eases up on the demands made to ceasefire groups, many of whom it holds only tenuous truces with.