Myanmar's junta likely used chemical weapons in a recent attack against ethnic rebels near the Thai border, an international human rights group said Friday.

London-based Christian Solidarity Worldwide (search) said it has "circumstantial evidence" that Myanmar (search) forces fired an artillery shell that emitted yellow smoke and a "highly irritating odor."

Within minutes of the Feb. 15 shelling, five ethnic Karen guerrillas near the explosion were stricken with lung and skin irritations and severe muscle weakness, said a report by CSW's Dr. Martin Panter, who interviewed and examined the rebels last week.

"Strong circumstantial evidence exists for the use of chemicals, particularly nerve agents, pulmonary agents and possibly blister agents," said the report on the group's Web site.

Myanmar junta officials weren't immediately available for comment.

Rimond Htoo, secretary general of the Karenni National People's Party, said the attack occurred at his group's camp about 9.3 miles inside Myanmar, near the Thai border town of Mae Hong Son.

"Five of our soldiers ... suffered strange injuries from the mortar shell," he told The Associated Press. "It was strange because of the sound of the explosion and the smoke."

He said the soldiers were sent to a hospital in Thailand after they vomited blood and fainted. Three have recovered, but two remain ill, he added.

"We do not know whether it was a chemical weapon," said Htoo.

Thai authorities were skeptical of the group's report.

Maj. Gen. Thanongsak Apirakyothin, commander of a Thai-Myanmar border security force, said "there is no information that Myanmar is using chemical weapons."

"I think the ethnic minority group is making this noisy complaint to draw attention from the international community," he told reporters.

CSW also called on the international community to step up pressure on the regime in Myanmar, also known as Burma, to improve its human rights record and block it from assuming the rotating chairmanship of the Association of Southeast Asian Nations next year.