Dec 27 (Reuters) - A senior U.N. official said he was horrified by the reported killing of at least 35 civilians in Myanmar and called on authorities to investigate the incident that opposition activists blamed on government soldiers. The ruling military has not commented on the incident near Mo So village in Kayah State on Friday and junta spokesman Zaw Min Tun could not be reached for comment. State media reported that soldiers had fired on and killed an unspecified number of "terrorists with weapons" from forces fighting the military government. State media did not say anything about civilian casualties. U.N. Under-Secretary-General for Humanitarian Affairs and Emergency Relief Coordinator Martin Griffiths said the reports of the killing of the civilians, including at least one child, were credible. "I condemn this grievous incident and all attacks against civilians throughout the country, which are prohibited under international humanitarian law," he said in a statement. Griffiths called for "a thorough and transparent" investigation so the perpetrators could be brought to justice and called for the protection of civilians. Residents and a human rights group working in the area said soldiers had killed the civilians. Photographs posted by the rights group showed charred bodies, some in the back of a burned-out truck. Myanmar has been in turmoil since the military on Feb. 1 overthrew the elected government of Nobel Prize laureate Aung San Suu Kyi. More than 1,300 people have been killed in crackdowns on protests and more than 11,000 have been jailed, according to a tally by the Association for Assistance of Political Prisoners rights group. The military disputes the group"s death toll. Some opponents of the military have taken up arms, some linking up with ethnic minority guerrillas who have for years been fighting the government for self-determination in various parts of the country, including Kayah State in the east. The Save the Children aid group said two of its workers, travelling to their home villages for the year-end holiday, had gone missing in the attack. It suspended operations in Kayah State and parts of neighbouring Karen State and the Magway region.