Mali's private media have launched a news blackout after an editor was arrested for publishing a letter about poor conditions from soldiers fighting Islamist militants in the north.Le Republicain editor Boukary Daou was arrested last Wednesday and has yet to be charged.The soldiers said they lacked equipment and rations - the same complaints made by those behind last year's March coup.Mali's interim government has not commented on the arrest or strike.'Unpatriotic'The BBC's Alex Duval Smith in the capital, Bamako, says about 40 newspaper titles are published each week - and none have appeared on news stands in the city on Tuesday morning.The 16 local private FM radio stations are either silent or only playing music, she says.Communications Minister Manga Dembele has said Mr Daou acted irresponsibly and unpatriotically by publishing the soldiers' open letter to the president, but there has been no official word from the authorities about the case.The letter also complained that the military top brass were living in comfort in Bamako while their colleagues suffered on the frontline.Capt Amadou Sanogo led the March 2012 coup saying the army was ill-equipped to fight a rebellion led by Tuareg separatist fighters in the north.Islamist rebels then took advantage of the chaos and seized control of the whole of northern Mali - prompting France to intervene in January 2013 to help the army recapture most of the area.Cap Sanogo was recently named the head of a government committee to oversee reforms in the military and is believed to be paid about $7,800 (£5,250) a month."Do we have to stage a coup to be rewarded and recognised as a good soldier?" the letter says.'Retaliatory attacks'Our reporter says that under Malian law, Mr Daou should have been charged within 48 hours of his detention.But Mali's legal landscape has been confused by the current state of emergency, she says.The striking journalists said they did not expect their colleagues working at the state-owned broadcaster and newspaper to join their indefinite protest.The journalists and media proprietors say they have faced a series of arbitrary arrests and beatings since the coup.Those operating in the north have also faced attacks from Islamist groups.Radio Guintan, a station for women, had all its transmitter towers in the region destroyed."The situation is chaotic for the media because there are people in authority who believe that if we're stopped from denouncing what they're up to, then they'll get away with it," Radio Guintan's Ramata Dia told the BBC.Global media watchdog the Committee to Protect Journalists (CPJ) said Mr Daou's arrest was "another example of Malian security agents acting outside the law in trying to harass journalists"."We call on the government to immediately release Daou," CPJ's Africa advocacy co-ordinator Mohamed Keita said in a statement.Meanwhile, the UN's human rights body said on Tuesday that its preliminary investigations show Malian soldiers have been carrying out retaliatory attacks on ethnic groups perceived to have supported rebel groups, the AFP news agency reports.The "situation has been exacerbated by the propagation of inflammatory messages, including through the media, stigmatising members of these communities, thousands of whom have reportedly fled out of fear of reprisal by the Malian army," the agency quotes deputy UN High Commissioner for Human Rights, Kyung-wha Kang, as saying.Troops from several African soldiers have been deploying to Mali to take over from the French-led mission in the run-up to elections due in July.