Anti-junta fighters said they found over 30 burnt bodies, including women and children, on a highway in eastern Kayah state where pro-democracy rebels have been fighting the military.

Myanmar has been in chaos since the February coup, with more than 1,300 people killed in a crackdown by security forces, according to a local monitoring group.

Self-proclaimed "People's Defence Forces" have sprung up across the country to fight the junta, and drawn the military into a bloody stalemate of clashes and reprisals.

Save the Children confirmed in a statement that two of its staffers, who had been missing, were "among at least 35 people, including women and children, who were killed."

"The military forced people from their cars, arrested some, killed many and burnt the bodies," it said, adding the two victims were both new fathers.

One of the men had been working as a teacher trainer and the other had joined the charity six years ago, the statement said, adding it would not identify them for security reasons.

"This news is absolutely horrifying," said chief executive Inger Ashing.

"We are shaken by the violence carried out against civilians and our staff, who are dedicated humanitarians, supporting millions of children in need across Myanmar."

The junta previously said its troops had been attacked in Hpruso township on Friday after they attempted to stop seven cars driving in a "suspicious way".

Troops killed an unspecified number of people in the following clash, spokesman Zaw Min Tun told AFP, without giving details.

The Myanmar Witness monitor said that satellite data showed a fire in the area at around 1:00 pm (0630 GMT) on Friday and that it had confirmed accounts that 35 people, including children and women, were burnt and killed by the military.

- US urges accountability -

The United States, which has taken the lead in sanctions over the coup, renewed calls for an arms embargo on the junta.

"The targeting of innocent people and humanitarian actors is unacceptable, and the military's widespread atrocities against the people of Burma underscore the urgency of holding its members accountable," US Secretary of State Antony Blinken said, using Myanmar's former name.

"The international community must do more to advance this goal and prevent the recurrence of atrocities in Burma, including by ending the sale of arms and dual-use technology to the military," he said in a statement.

Western nations have long restricted weapons to Myanmar's military, which even during the pre-coup democratic transition faced allegations of crimes against humanity for a bloody campaign against the Rohingya minority.

The UN General Assembly voted in June to prevent arms shipments into Myanmar but the measure was symbolic as it was not taken up by the more powerful Security Council.

China and Russia, which hold veto power on the Security Council, as well as neighbouring India are the major arms providers to Myanmar.

United Nations under-secretary-general for humanitarian affairs Martin Griffiths earlier said he was "horrified" by reports of the massacre and demanded that authorities conduct an investigation.

Save the Children, which has a staff of around 900 working in Myanmar, suspended operations in Kayah state and several other regions.

In October the group said its office in the western town of Thantlang was destroyed in junta shelling that also razed dozens of homes following clashes with a local anti-junta group.