Anti-coup protesters returned to the streets across Myanmar on Thursday, with some burning copies of the 2008 Constitution that guarantees the military's role in politics, a day after pro-democracy forces announced a provisional charter in its place.

Myanmar's deposed leader, Aung San Suu Kyi, has been charged with breaking a colonial-era official secrets law, her lawyer said on Thursday, the most serious charge against the veteran opponent of military rule.

On Saturday, the KNU condemned the use of "excessive force by engaging in non-stop bombing and air strikes" from March 27 to 30, which have "caused the deaths of many people, including children".

Junta spokesman Zaw Min Tun said the military has only been targeting the KNU 5th Brigade - which led the seizure of the military base and killed officers.

The U.N. Human Rights Office for Southeast Asia called on countries in the region "to protect all people fleeing violence and persecution in the country" and "ensure that refugees and undocumented migrants are not forcibly returned", U.N. spokesman Stephane Dujarric told reporters in NY.

It comes after the Security Council concluded its meeting Wednesday with no immediate sign of any agreement on more aggressive actions on Myanmar. "If they follow the NCA, there is no reason for conflict to happen", Zaw Min Tun said.

About 3,000 people fled to neighbouring Thailand on Monday, crossing the Salween River to seek shelter. Thai authorities said they went back voluntarily after a brief stay, but aid groups say they are not safe and many remain in hiding in the jungle and in caves on the Myanmar side of the border.

Fighting has also flared in the north between the army and ethnic Kachin insurgents.

Meanwhile, the Shan State Refugee Committee has issued a statement saying it is "gravely concerned" for the safety of almost 6,000 internally displaced people in camps along the southern Shan State-Thai border, following the Tatmadaw'a announcement that it will start attacking border positions of the Restoration Council of Shan State/Shan State Army.

Protests - and the security forces' tough response - continue and in Monywa in central Myanmar on Thursday afternoon a 31-year-old protester was shot dead, while 10 others were wounded, a rescue worker told AFP. Access to mobile networks and all wireless - the less costly options used by most people in the developing country - was blocked.

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The military is also stepping up its efforts to crush dissent with arrest warrants for online critics.

A conviction under the law can carry a prison sentence of up to 14 years.

Her lawyers have said the charges she faces were trumped up.

"Whether a warrant has been issued or not, as long as I'm alive I'll oppose the military dictatorship who are bullying and killing people".

Paing Phyoe Thu regularly attended rallies in the main city of Yangon in the weeks after the coup.

Facebook announced it was providing a safety feature to enable users in Myanmar to beef up security settings locking their profiles to prevent access by non-friends. Some of those banned or whose operations have been suspended have continued to publish via social media or whatever methods they can find.

Over 500 people have been killed in Myanmar since the military seized power in the February 1 coup, ousting Suu Kyi's elected government, with security forces increasingly resorting to the use of deadly force to quell a wave of protests.

The U.N. Human Rights Office for Southeast Asia called on countries in the region on Thursday "to protect all people fleeing violence and persecution" in Myanmar and "ensure that refugees and undocumented migrants are not forcibly returned given the rapidly deteriorating human rights situation" there, U.N. spokesman Stephane Dujarric told reporters at the world body's headquarters in NY. One police officer, who was carrying a gun, asked if "anyone dared to help them", a witness told the news service. "He died on the spot".

Reuters could not immediately verify the reports and a junta spokesperson did not answer calls seeking comment.