GENEVA, Switzerland - Myanmar should investigate reports - denied by the government - that inmates of a Yangon prison were shot dead as Cyclone Nargis ravaged the country last month, a U.N. human-rights expert said yesterday.

Tomas Ojea Quintana, the U.N. Human Rights Council's new investigator for Myanmar, said security forces apparently opened fire after inmates at Insein prison panicked when the facility's zinc roofs were blown off during the May 2-3 cyclone.

"The authorities should conduct a thorough and transparent investigation to clarify the facts and identify the perpetrators of these arbitrary killings," he said in a 16-page report to the rights council.

Tate Naing of the Assistance Association for Political Prisoners (Burma) put the death toll at 40, based on several reports the group received.

The Thailand-based organization for Myanmar exiles said more than 1,500 prisoners were forced to congregate inside a prison hall and were locked inside until the morning of May 3.

Angry prisoners set fire to the hall and a riot ensued. Guards attempting to contain the situation opened fire, and soldiers and riot police were called in, the group said.

Myanmar's government rejected the claim that inmates had been shot in the incident.

"No one was killed or injured during the event," said Wunna Maung Lwin, Myanmar's ambassador to the U.N. office in Geneva. "The prison security, as well as the police and the military, had not in any circumstances used arms against the prisoners."

Quintana, who took up his post May 1, has yet to visit the country himself.

Beyond the cyclone, Quintana said he was greatly concerned that the country's rulers had made no improvement on important issues such as the treatment of political prisoners, freedom of expression, and freedom of assembly.

"The situation of human rights in Myanmar . . . has not changed for the better," said Quintana, a rights expert from Argentina.

"The reported number of political prisoners and detention conditions continue to be appalling," Quintana said, adding he had been informed that 1,900 people are imprisoned on political grounds.