Britons have been advised to leave Myanmar as violence in the southeast Asian country continues after a military coup.
Twelve more protesters were killed on Thursday, taking the total to more than 70, according to Assistance Association for Political Prisoners (AAPP).
UN experts say evidence is mounting that crimes against humanity could be taking place.
Violence has escalated since the army took control on 1 February, declaring a state of emergency and detaining leader Aung San Suu Kyi and party officials.
The UK Foreign Office said British nationals should leave Myanmar "by commercial means, unless there is an urgent need to stay".
"Figures in the Civilian Government, civil society and a foreign national have been detained by the military," said a statement.
"Political tension and unrest are widespread since the military takeover and levels of violence are rising."
It also cited a nightly internet shutdown, problems with phone networks, a nationwide curfew between 8pm and 4am, and difficulty getting cash.
The Foreign Office said if people couldn't leave they should "stay home and stay safe" and avoid any crowds if they do go out.
Thursday's violence was among the worst since the military seized power and came hours after a call from the UN Security Council for restraint.
Eight of the 12 killed were protesters who were shot in the central town of Myaing, said the AAPP.
UN experts believe there is now "growing evidence" the military regime is "likely engaging in crimes against humanity".
Human rights special rapporteur Thomas Andrews said on Thursday that crimes may include murder, enforced disappearance, persecution, torture, and unlawful imprisonment.
"There is shocking video of the aftermath of attacks, including fatal gunshot wounds to the heads of protesters, and video of soldiers dragging or carrying away the dead bodies of their victims," Mr Andrews told the UN Humans Right Council.
He said there were "credible reports" that security forces had murdered at least 70 people.
Myanmar's army has also made new accusations against detained leader Ms Suu Kyi.
It claims she accepted 11kg gold and illegal payments of $600,000 while in government - dismissed by her lawyer as a "hilarious joke".
The army justified their power grab by claiming there was fraud in November's election - won by Ms Suu Kyi - but Myanmar's electoral commission has rejected that assertion.
The country's state of emergency will last for a year, according to the military.
It says it will only be in charge until fresh elections are held, but so far hasn't set a date for a new vote.