The United Nations human rights expert on Myanmar has said Russia and China were providing the junta with fighter jets being used against civilians.
The UN expert on Tuesday also urged the UN Security Council to halt the flow of weapons enabling atrocities, reports Reuters.
Thomas Andrews, a former US congressman serving in the independent post, released a report that also named Serbia as one of three countries supplying arms to the Myanmar military since it seized power last year, with "full knowledge that they would be used to attack civilians".
"It should be incontrovertible that weapons used to kill civilians should no longer be transferred to Myanmar," Andrews said in a statement.
Chaos has gripped Myanmar since a coup ended a decade of tentative democracy and triggering nationwide protests that troops suppressed with lethal force.
At least 1,500 civilians have been killed, according to activists cited by the UN, which also says more than 300,000 people have been displaced by rural conflict between the military and armed opponents.
The junta says it is fighting "terrorists" and objects to what it calls UN interference.
Myanmar's military and the foreign ministries of China, Russia, and Serbia could not immediately be reached for comment on the report.
Human rights groups and the UN have accused the junta of using disproportionate force to fight militias and ethnic minority rebels, including artillery and air strikes in civilian areas.
The report said Russia had supplied drones, two types of fighter jets, and two kinds of armoured vehicles, one with air defence systems. China transferred fighter jets while Serbia had provided rockets and artillery shells, it said.
The UN General Assembly adopted a resolution last year calling on members to halt arms transfers to Myanmar's military, which Andrews said the security council should make binding.
Serbia voted in favour of the resolution, but Russia and China abstained.
While China has urged an end to hostilities in Myanmar, Russia has been the generals' closest diplomatic ally amid efforts by the West to isolate them.
Andrews also called for cutting the Myanmar military's access to oil and gas revenue and foreign exchange reserves, plus international bans on purchases of Myanmar timber, gemstones, and rare earths.
Myanmar's rulers were vulnerable and could be stopped with international resolve, he said in the report.
"If revenues necessary to maintain such a military are reduced, the junta's capacity to assault and terrorise the people of Myanmar will diminish," he said.