Myanmar cut internet service and deployed troops around the country on Monday in signs of a feared crackdown on protests, hours after security forces fired to disperse a demonstration in the country's north.

The State Administration Council has escalated efforts to quell a burgeoning civil disobedience campaign which is demanding a return of Aung San Suu Kyi.

Monday's internet shutdown and a request from the United Nations for an observer to be allowed in came soon after live-stream images shared on social media platforms showed military vehicles and soldiers moving through some parts of the country.

Monitoring group NetBlocks said the "state-ordered information blackout" had taken Myanmar almost entirely offline.

Troops in Myitkyina fired tear gas then shot at a crowd who gathered in the northern city to stop a rumoured shutdown of the electricity grid.

A joint statement from the US, British and European Union ambassadors urged security forces not to harm civilians.

"We call on security forces to refrain from violence against demonstrators, who are protesting the overthrow of their legitimate government," they said.

UN Secretary-General Antonio Guterres echoed that call, pushing authorities to "ensure the right of peaceful assembly is fully respected and demonstrators are not subjected to reprisals".

Through his spokesman, Guterres also asked the military to "urgently" allow Swiss diplomat Christine Schraner Burgener to visit Myanmar "to assess the situation first hand".

The US embassy advised American citizens to shelter in place and not risk defying an overnight curfew imposed by the State Administration Council.