Save the Children on Tuesday confirmed the deaths of two of its workers who had been caught up in the incident.
Myanmar has been in chaos since a February coup against the elected government, with more than 1,300 people killed in a crackdown by security forces, according to a local monitoring group.
The United States has slapped a series of sanctions on the coup leaders and like other Western nations has long restricted weapons to Myanmar's military, which during the pre-coup democratic transition faced allegations of crimes against humanity for a brutal 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 neighboring India are the major arms providers to Myanmar.