Cyclone Nargis slammed into Myanmar's Irrawaddy Delta on May 2 last year, leaving an estimated 140,000 people dead or missing and severely affecting 2.4 million others.

After the ruling junta was slow to mobilize rescue efforts and initially barred most foreign assistance, local community groups started providing aid on their own initiative. In several cases, groups and individuals already critical of the junta took part.

New York-based Human Rights Watch said in a statement that at least 21 people involved in such aid efforts have been "unfairly tried and imprisoned," including Zarganar, a famous comedian and veteran social and political activist who is serving a 35-year prison term.

It said that without the work of community groups and religious organizations, including Buddhist temples and Christian charities, "the disaster response would have cost many more lives."

Zarganar was convicted of causing public alarm and illegally giving information to foreign media, and was originally given a 59-year sentence.

A Thailand-based advocacy group, the Assistance Association for Political Prisoners (Burma), issued a report Friday about 21 people imprisoned for between two and 35 years after helping cyclone victims. Myanmar is also known as Burma, a term often used by groups which consider the ruling military regime illegitimate.

"Their punishment is completely unacceptable," one of the group's leaders, Tate Naing, said in a statement. "Their 'crimes' were to help people and tell the truth about the situation."

At least eight of the volunteers who helped out had been in hiding since participating in September 2007 pro-democracy demonstrations that were suppressed by the army. Five are former political prisoners who have already spent many years in jail, it added.

Myanmar's military, which has held power since 1962, tolerates little dissent. There are about 2,100 political prisoners in Myanmar, according to the U.N. and international human rights groups. The government denies holding any political prisoners, saying that all convicts have been imprisoned for criminal violations.

"Basic freedoms for cyclone survivors are just as restricted as they were before the cyclone," said Elaine Pearson, deputy Asia director at Human Rights Watch.