Myanmar President Thein Sein freed 73 political prisoners last week. More than 600 were released last year as part of a political reform effort, which began with general elections in 2010.
Tomas Ojea Quintana, a U.N. special envoy on the rights situation in Myanmar, said the prisoner release was commended as part of the country's general reforms process.
"However, I am very concerned about ongoing arrests and sentencing," he said in a statement Monday. "I believe there are still arrests and trials taking place that are politically motivated."
He said he would raise the issue of land confiscation and arrests of those working to address the human rights situation in Rakhine when he visits Myanmar in August.
Quintana welcomed a recent decision by Myanmar's government to disband the Nasaka border security force, which he said was suspected of committing human rights violations in Rakhine, an western coastal state.
Rights groups say Myanmar has a long road ahead despite political reform efforts. Western governments, however, have eased sanctions on Myanmar in recognition of the reforms.