Intense fighting in eastern Myanmar between government troops and ethnic guerrillas has killed dozens and threatens to spill over into neighbouring Thailand.
The Myanmar army and its allies in the United Wa State Army have been attacking positions held by a rival ethnic group, the Shan State Army (SSA), opposite Thailand's Chiang Mai province, in a battle for territory and for control of the drugs trade.
Thai officials said more than a dozen shells fired by one or both sides had landed inside Thailand since Tuesday and many Shan army troops had retreated across the border for medical treatment.
They said dozens of Myanmar and Wa fighters died in the assaults on Shan positions. Myanmar had sought without success Thai permission to cross the border to hit the Shan from behind.
"We are on full alert for any incursion into Thailand and we are reinforcing positions along the border that may be at risk," a Thai defence ministry spokesman said.
An Shan source said his forces had killed about 150 Myanmar and Wa soldiers and wounded many others in heavy fighting along the border since May 20. He declined to give an estimate for the Shan's casualties.
Myanmar officials were unavailable for immediate comment.
Tensions have flared between Thailand and Myanmar, with Myanmar accusing Thailand of backing the Shan and another ethnic army, the Karen National Union (KNU), a charge Bangkok denies.
Myanmar has sealed major border crossings and this week warned Thailand not to get involved in its assault on the Shan.
Thailand's government has tried to defuse the mounting tension and has said Myanmar troops are not a threat as they are fighting rebel groups and not launching a strike into Thai soil.
Thailand has said it wants high-level talks with Myanmar to calm tensions, but so far has received no reply.
The two countries have been at odds in recent years over the flow of illegal narcotics from factories inside Myanmar.