The killings took place on Christmas Eve in eastern Kayah state, where pro-democracy rebels have been fighting the military, which took over the government from the democratically elected administration in February.

The EU foreign policy chief Josep Borrell said "the appalling act of violence perpetrated by the military regime" on civilians and humanitarian workers underlined the "urgent need" to hold the junta accountable.

"In view of the escalating violence in Myanmar, increased international preventive action is required, including an arms embargo," Borrell said in a statement.

"The EU also stands ready to impose further sanctions against the military regime," he added.

His call for an international arms embargo echoed one Tuesday from the United States.

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.

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

Since the coup, the EU has imposed targeted sanctions on the Myanmar military, its leaders and entities.

The bloc also halted EU financial assistance to the government and froze assistance that could be seen as legitimising the military regime.

Borrell said "the targeting of civilians and humanitarian actors is unacceptable and a blatant violation of human rights and international law, including humanitarian law."

He called for "full, safe and unhindered humanitarian access to" Myanmar's people while demanding full protection for humanitarian workers and medical personnel.

International charity Save the Children said two of its employees were among those killed in the massacre.

The EU said it will continue providing humanitarian aid to the people.