Myanmar's ousted civilian leader Aung San Suu Kyi has been accused of breaking a colonial-era official secrets law, her lawyer said, as Britain ramped up sanctions against the junta and the UN Security Council condemned the deaths of hundreds of civilians.

The newly unveiled charge came amid growing international outrage over the February 1 coup and the military's subsequent clampdown on protesters that has left at least 535 people dead.

After two days of back-and-forth negotiations with China, Russia and the rest of the Security Council, members on Thursday unanimously "expressed deep concern at the rapidly deteriorating situation" in Myanmar.

More than 535 people have died in daily demonstrations
More than 535 people have died in daily demonstrations KAREN INFORMATION CENTER / Handout

The Council "strongly condemned the use of violence against peaceful protestors and the deaths of hundreds of civilians, including women and children," they said in a statement, initiated by former colonial power Britain.

London earlier announced sanctions on the Myanmar Economic Corporation (MEC), a conglomerate controlled by the military that Washington has already blacklisted.

In addition to sanctions, London will also stump up $700,000 towards UN Security Council efforts to document serious human rights violations in Myanmar.

The coup and the junta's subsequent actions have triggered international condemnation and even sanctions
The coup and the junta's subsequent actions have triggered international condemnation and even sanctions FACEBOOK / Handout

"Two months on from the start of the coup, the Myanmar military has sunk to a new low with the wanton killing of innocent people, including children," British foreign minister Dominic Raab said in a statement.

International powers have sought to pile pressure on the military by hitting its sprawling business interests, which include the lucrative jade and ruby trade.

Earlier, Suu Kyi appeared by video link in court in the capital Naypyidaw, where she faces a raft of charges that could see her barred from political office.

Protesters around Dawei in southern Myanmar march and ride motorbikes in order to avoid confrontation with security forces
Protesters around Dawei in southern Myanmar march and ride motorbikes in order to avoid confrontation with security forces DAWEI WATCH

The hearing dealt with administrative aspects of the case including the formal appointment of eight defence lawyers.

"Daw Aung San Suu Kyi's physical condition was good," according to the lawyer who saw her on screen, her legal representative Khin Maung Zaw told reporters.

He later told AFP that the Nobel laureate has been accused of breaking an official secrets law in a lawsuit filed March 25.

The military-owned Ruby Mart supermarket in Yangon was set on fire overnight
The military-owned Ruby Mart supermarket in Yangon was set on fire overnight AFP / STR

Internet firms were ordered to cut wireless services, provider Ooredoo said, in the latest move to suppress communication.

Civilian leader Aung San Suu Kyi has not been seen in public since she was detained by the military on February 1
Civilian leader Aung San Suu Kyi has not been seen in public since she was detained by the military on February 1 AFP / STR

Dozens of UN member states subsequently issued a statement denouncing such shutdowns and attacks on freedom of expression, expressing "deep concern" over the plight of media workers.

A group of ousted MPs from Suu Kyi's National League for Democracy (NLD), who have been working underground against the junta, have announced plans for "a new civilian government" in the first week of April.

They said Myanmar's military-drafted 2008 constitution was "cancelled", and on Thursday a group of protesters burned a pile of copies in the street in Yangon.

In another blow to the junta's business interests, two military-owned supermarkets in Yangon were set ablaze overnight, and two more international companies cut ties because of the crisis.

Protests -- and the security forces' tough response -- continue, and in Monywa in central Myanmar on Thursday a 31-year-old protester was shot dead, while 10 others were wounded, a rescue worker told AFP.

One person was also killed, and six others injured, in Mandalay, a rescue worker and a doctor said.

And the International Federation of Red Cross and Red Crescent Societies said security forces had been targeting first responders, with Asia-Pacific director Alexander Matheou labelling the incidents as "unacceptable".

Fears are also growing that a broader conflict could erupt in a country plagued for decades by on-and-off fighting between the military and rebel ethnic armies.

Several of Myanmar's 20 or so armed ethnic groups, who control large areas of territory mostly in border regions, have voiced their opposition to the coup and crackdown.

Two other outfits -- the Karen National Union (KNU) and the Kachin Independence Army (KIA) -- have already stepped up attacks on military and police in recent days.

In another escalation, since Saturday the military has launched regular air strikes targeting the KNU in eastern Karen state.

Local media outlet Karen News reported that 11 people were killed in an air strike in a gold mining area of the state on Tuesday. AFP has yet to independently confirm those details.