European Union diplomatic chief Josep Borrell on Sunday condemned a military crackdown in Myanmar and confirmed the bloc would adopt sanctions in response.

"Violence will not give legitimacy to the illegal over-throwing of the democratically-elected government" in Myanmar, Borrell said in a statement.

"In shooting against unarmed citizens, the security forces have shown a blatant disregard for international law, and must be held to account."

The UN rights office said at least 18 people were killed, several wounded and hundreds arrested Sunday when security forces dispersed pro-democracy demonstrations in Myanmar.

It marks the bloodiest action so far to smother opposition to the country's military coup four weeks ago.

"The European Union will take measures in response to these developments shortly," Borrell said.

European ministers have already agreed on sanctions against Myanmar's military over the coup and have decided to withhold some development aid.

The sanctions are expected to be finalised in the coming days and will take effect once an official notice has been published by the EU.

© AFP