"Than Shew's regime continues to arrest, prosecute and imprison peaceful political activists," a U.S. State Department statement said. "No referendum held under these conditions ... can be free, fair or credible."
The military government of Myanmar, formerly known as Burma, recently announced plans to conduct general elections in 2010 and work toward a new constitution beginning in May.
"The Burmese junta's announcement that it will hold a referendum on a new constitution in May demonstrates its lack of seriousness about an open and fair process for the restoration of democracy," White House Press Secretary Dana Perino said during a media briefing.
Perino said the constitution's drafting process neither incorporated opposition views nor allowed enough time for adequate debate. Also, the constitution "has not yet been shared with the Burmese public," she said.
The move toward a democratic government in Myanmar follows more than four decades of military rule.
Florida bear attack: Black bear mauls woman's face
Martin Bashir resigns from MSNBC over Sarah Palin comments