Yangon - Rights groups welcomed the release of a prominent Rohingya Muslim doctor who was arrested while trying to calm rioters during sectarian violence in western Myanmar, but noted on Tuesday scores of political prisoners remain behind bars.The case against Tun Aung, sentenced to 17 years in prison following what was considered an unfair trial, received widespread international attention.He was accused of inciting violence between Rohingya Muslims and Rakhine Buddhists in June 2012, though rights groups and witnesses said the medical doctor and respected community leader was asked by authorities to try to intervene but was unable to stop the violence.Bo Kyi, co-founder of the Assistance Association of Political Prisoners (AAPP), said Aung was released on Monday.Myanmar has freed more than 1 000 political prisoners since former military rulers handed over power four years ago, a move that has smoothed the former pariah state's international rehabilitation. But jails continue to be filled up with hundreds of peaceful protesters, journalists and farmers who stood up against land grabs by the rich and powerful."Everyone incarcerated for their beliefs should be freed unconditionally and immediately," said Aung Myo Min, executive director of Equality Myanmar, adding the government "releases a few political prisoners, and then arrests a few more."It's like a "hostage-swap," he said.Though years were sliced from Aung's sentence through regular amnesties and presidential pardons, international pressure appeared to provide the the final push.UN Special Rapporteur on Myanmar, Yanghee Lee, met Aung in Insein prison during her visit to the country last week.Tom Malinowski, a senior US human rights envoy, whose own trip coincided with Lee's, told Myanmar that no country wants to have the reputation of locking people up solely for peacefully protesting or expressing their views.He also noted that laws appear to be unevenly applied.No efforts were made to curtail extremist Buddhist monks who marched through the streets during Lee's visit, spewing hatred and accusing her of being biased in favour of Rohingya.Myanmar has been grappling with rising Buddhist nationalism since former military rulers handed over power to a nominally civilian government in 2011, largely flamed by radical monks. Violence has left more than 240 people dead and sent another 140 000 fleeing their homes — most of them members of the Rohingya minority.