
"The Thai government cynically launched this illegal operation during the first day of a major Buddhist holiday," said Brad Adams, Asia director at Human Rights Watch.
Thailand is a predominantly Buddhist country.
"This, along with the Thai media's preoccupation with escalating border tensions with Cambodia over the Preah Vihear temple, provides effective cover for Thailand's serious breach of international law," said Adams in a statement issued from New York.
Thai media attention has been focused this week on the Thai-Cambodian border, where Thai and Cambodian troops have amassed over an escalating row over the ancient Hindu temple Preah Vihear, perched on the border and subject to a territorial dispute.
Thailand, a magnet for hundreds of thousands of refugees and illegal workers from its less developed neighbours - Cambodia, Laos and Burma - has not signed the 1951 Refugee Convention, but Adams argued that Bangkok is still bound by the principle of non-refoulement, a prohibition in customary international law, from returning refugees to any country where they are likely to be persecuted or their lives are at risk.
"The Thai government has ignored its obligations to protect refugees fleeing persecution and violence in Burma," Adams said. "Sending these people back over conflict zones dominated by the Burmese army is disgraceful."
The Karen, an ethnic minority groups whose traditional territory is the Karen State of eastern Myanmar, have been under attack by the Burmese military for decades in what some have termed a genocidal campaign.
The Karen National Union (KNU) is one of the few Burma-based insurgencies that has refused to sign a ceasefire agreement with Burmar's ruling junta. They have been fighting a war for the autonomy of their state since 1949.
"Forcing civilians back into an active war zone may be an easy answer for Thailand, but it's brutal - a completely inhumane and unacceptable solution," said Adams.
Human Rights Watch called on the United Nations High Commissioner for Refugees (UNHCR), the European Union, the United States and other countries to pressure the Thai government to immediately cease the refoulement of refugees and continue to provide sanctuary to people fleeing fighting or persecution in Burma.