The Kachin Independence Army (KIA) said it downed the helicopter gunship during fierce clashes near the town of Momauk in the country's far north.

Myanmar has been in turmoil since the military ousted civilian leader Aung San Suu Kyi and seized power on February 1.

Security forces have sought to quell near-daily pro-democracy protests with deadly force, while long-simmering conflicts with ethnic rebels have erupted into fierce fighting.

The KIA, which has waged a decades-long insurgency against the military in northern Kachin state, has been hit by air strikes in recent weeks.

"They used jet fighter and helicopter gunship since eight this morning to attack our troops. Our troops fired back and this was how the helicopter was shot down," said KIA spokesman Col Naw Bu, who refused to give details of the weaponry used to down the chopper.

AFP has tried to contact the Myanmar military to seek their account of events, but has not had a response.

A number of Myanmar's myriad ethnic rebel groups have come out in support of the anti-coup protest movement, offering shelter and even training to activists who have fled the crackdown.

The Assistance Association for Political Prisoners (AAPP), a local monitoring group, said that at least five civilians were killed in operations to break up protests on Sunday.

AAPP said it was the highest single-day death toll since an ASEAN summit on the Myanmar crisis last month.

A total of 765 civilians have been killed in the crackdown, according to the AAPP, though the military disputes the toll, giving a much lower figure.

More than 4,500 people have been arrested, the monitor has said, including dozens of journalists.

On Monday -- World Press Freedom Day -- a group of foreign embassies in Myanmar condemned the junta's treatment of journalists, saying they had become a "target of repression".

Independent media outlets have been shut down or had their licences revoked, and Myanmar authorities have throttled internet access in an effort to stem the flow of information about the protests and crackdown.

"We call for the immediate release of all media workers, the establishment of the freedom of information and communication and for the end of all internet restrictions in Myanmar," said the statement, noting that of 80 journalists arrested by the authorities, more than half were still detained.

The signatories included the embassies of the United States, the European Union, Australia, Britain, France and Germany.