YANGON (Reuters) - Myanmar’s military opened legal proceedings on Wednesday against three journalists accused of aiding ethnic minority rebels, police said, compounding concern about the erosion or press freedom as the country transitions towards civilian rule.

The three, Aye Nai and Pyae Phone Naing of Democratic Voice of Burma (DVB) and Lawi Weng, a senior reporter with the Irrawaddy magazine, were taken into military custody in Shan State in northeast Myanmar on Monday.

They had covered the burning of some drugs by the rebel Ta’ang National Liberation Army (TNLA), which demands more autonomy for the Ta’ang ethnic group and has been engaged in clashes with government troops over the past week.

Deputy police inspector Kyaw Min said the three reporters, along with three other men detained at the same time, had been brought to police in Hsipaw town, where they would be imprisoned.

“Today, in the afternoon, the military opened the case under Article 17(1) of the Unlawful Association Act,” said Kyaw Min.

The legal article, which prohibits giving assistance to banned groups, carries a penalty of up to three years in prison.

“We will have to propose the case at the court and will detain them at Hsipaw prison,” he added.

Despite the end of full military rule, and the election in 2015 of a government led by democracy champion Aung San Suu Kyi, concern in Myanmar is growing about press freedom, and about the challenge Suu Kyi faces in achieving peace.

Clashes between government troops and loosely allied rebel groups close to Myanmar’s border with China have set back peace talks prioritized by Suu Kyi when she took power last year.

The DVB and the Irrawaddy, which operate in English and Burmese, were formed in exile during military rule but have both set up bureaus in the city of Yangon since pre-publication censorship was officially abolished in 2012.

Domestic media advocacy groups and international watchdogs, including Amnesty International, raised concern about the reporters.

‘AFFRONT TO DEMOCRACY’

Lawyer Nyan Win, a senior member of Suu Kyi’s National League for Democracy (NLD) party, raised concerns about due process, although he said he was not sure of the full details of the case.

“There’s no provision for detaining anybody secretly -there’s no clause in the law. And there’s no arresting power for the military,” he told Reuters when asked about the journalists’ detention.

The Myanmar Press Council has written to the Ministry of Defense calling for their release, according to a defense official. The official declined to elaborate.

The Committee to Protect Journalists said the three should be released and allowed to carry on with their work.

“Using the archaic Unlawful Associations Act to incarcerate journalists is an affront to democracy in Myanmar,” the group’s Asia Program Coordinator Steven Butler said in a statement.

Though she is de facto government leader, Suu Kyi does not have oversight over the armed forces. But she has not used her party’s parliamentary majority to repeal many of the laws used by previous regimes to stifle dissent.

Nyan Win said the NLD was reviewing such legislation.

Journalists demonstrated in Yangon this month against what they said were increasing curbs on free speech, especially the use of a telecommunications law to prosecute reporters and social media users for defamation.

The state-run Global New Light of Myanmar newspaper said the army - known as the Tatmadaw - had clashed five times with insurgents since the army discovered a TNLA training camp on June 20.

“Several Tatmadaw officers and troops were killed and a few were wounded. Four bodies of enemy troops were also found,” the newspaper said, citing military officials.

The clashes took place while the three journalists were in the TNLA zone for the drug-burning.

Two community groups in the area said on Tuesday they had documented extrajudicial killings, mass detention of civilians and torture by the military in recent days. They also said the military killed two villagers and wounded one in attack on Monday.

The Global New Light of Myanmar said rebels were responsible for the attack

Military spokesmen were not immediately available for comment. The army almost always rejects accusations of rights abuses although it has in recent years acknowledged occasional violations by soldiers and promised action.