A senior UN official called on Myanmar authorities to investigate the reported killing of at least 35 civilians that opposition activists blamed on government soldiers, saying he was "horrified" at the violence.

The ruling military has not commented on the killings near Mo So village in Kayah State on Friday and junta spokesman Zaw Min Tun could not be reached by phone for comment.

State media reported on Sunday 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 civilians.

U.N. Under-Secretary-General for Humanitarian Affairs and Emergency Relief Coordinator Martin Griffiths said reports of the killing of civilians, including at least one child, were credible.

"I am horrified by reports of an attack against civilians... 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.

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 February 1 overthrew the elected government of Nobel Prize laureate Aung San Suu Kyi.

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.

On Monday, three sources in the western Thai city of Mae Sot, 330 km (205 miles) to the south, said there were further signs of fighting on the Myanmar side of the border, including gunshots, plumes of smoke and an air strike.

According to Thai authorities, 5,260 refugees have fled the area into Thailand since a flare-up in fighting between the Myanmar army and the Karen National Union (KNU) insurgent group on December 16.

Thailand's foreign ministry spokesman said Thailand was working with the UN High Commissioner for Refugees (UNHCR), in case "Thai authorities are unable to manage the situation", following calls by aid organisations for Bangkok to do more to help.

Since Myanmar's military launched the coup, 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.