UK PM 'horrified' by escalation of Myanmar violence
"We stand with the people of Myanmar in calling for an immediate end to military repression, the release of Aung San Suu Kyi and others, and the restoration of democracy," Johnson said on Twitter, referring to Myanmar's de-facto leader who was detained in a February coup.
Johnson also condemned the killing of dozens of pro-democracy protesters on Wednesday in the deadliest day since the military takeover and the subsequent crackdown.
Britain, the former colonial power in Myanmar, has repeatedly condemned repression and rights abuses by the junta after leading generals swiftly ended a decade-long experiment with democracy.
The takeover triggered mass uprisings against the army generals, which have been met with deadly force.
Last week, Britain imposed sanctions on six members of Myanmar's military junta, including commander-in-chief General Min Aung Hlaing.
The sanctions will prevent the six from travelling to Britain, while British businesses and institutions are barred from dealing with them.
The same curbs had already been imposed on 19 other military figures.
UK aid that could be used to indirectly support the junta has also been suspended.