YANGON (AFP) - Myanmar pardoned hundreds of prisoners Thursday under a mass amnesty criticised by pro-democracy activists as a cynical ploy to curry favour ahead of a landmark visit by US President Barack Obama. Officials said 452 prisoners would walk free, but by the late afternoon rights groups said they were not aware of the release of any of the scores dissidents still languishing in jail. Myanmar, formerly known as Burma, has already freed hundreds of political prisoners incarcerated by the former junta, as part of reforms that have led to a dramatic thaw in relations between the one-time pariah nation and the West. Campaigners including New York-based Human Rights Watch cast doubt on the motivation for the latest amnesty.