BANGKOK, June 20 (UPI) -- Heavy fighting between Myanmar army forces and ethnic Shan rebels spilled over Thursday into Thailand, triggering an exchange of mortar fire between the Thai and Myanmar armies, officials said.
Thai officers said forces of the Myanmar junta had intensified their monthlong offensive against Shan State Army rebels, and five mortar rounds exploded on the Thai side of the border in Vieng Hang district of Chiang Mai province, 400 miles north of Bangkok.
Thai troops retaliated by firing their own mortars at the source of the firing within Myanmar, officers said.
Relations between Bangkok and Yangon have plunged to new lows during the past month, after Myanmar military authorities accused the Thai army of providing tank and artillery support to Shan rebels when they captured four Myanmar army bases along the border on May 20.
Two days later Myanmar shut down its border with Thailand, expelled Thai workers and cut all high-level diplomatic contacts.
No one was injured on the Thai side during Thursday's clash, but Wednesday two Thai army rangers were wounded by mortar fire from Myanmar troops firing at ethnic Karen rebels.
The fighting Wednesday forced some 300 Thai villagers to flee their homes in the Thai border province of Tak, 220 miles northwest of Bangkok.
In Bangkok, Thai Defence Minister Chavalit Yongchaiyudh played down the border skirmishes, describing the cross-border shelling by Myanmar forces as "accidental".
He dismissed local press reports that Myanmar forces were using chemical weapons, including the defoliant Agent Orange.
"I don't believe Burma would use chemical weapons against the Shan," he said. "If they used chemical weapons, it would be a violation of international agreements."
Chavalit and Prime Minister Thaksin Shinawatra have been accused of treating frequent border violations by Myanmar forces too leniently.
Thaksin advised Thai troops not to "over-react" to provocations from the Myanmar junta.
Critics have pointed to the extensive business in Myanmar of his family business, the Shin Corp., as constituting a conflict of interest with his role as prime minister.
Rebel sources said fighting was likely to become more fierce in the next month as Myanmar gears up to capture a string of Shan rebel bases and the headquarters of the Karen National Union at Walekee, opposite the Thai town of Mae Sot.
They said Myanmar forces were relying heavily on artillery and 120-mm mortars purchased from China.
The ethnic Shan and Karen rebels have been fighting for independence from the central government in Yangon, formerly Rangoon, since 1962 when the ethnic Burman-dominated army seized power.
The country has been run by a series of military dictatorships since then. The current junta took power in 1988, changed the name of the country from Burma to Myanmar and set out to subdue all ethnic rebel resistance.