Myanmar's main opposition movement estimates that at least 330 political prisoners remain incarcerated, according to Nyan Win, a spokesman for Aung San Suu Kyi's National League for Democracy party.
Nyan Win said he believes the latest release is "a goodwill gesture" ahead of Obama's trip. "We want all political prisoners to be freed," he added.
New York-based Human Rights Watch accused the government of using strategically timed prisoner releases to appease the international community.
"The government of Burma has said they are committed to releasing all political prisoners. So why haven't they?" said Phil Robertson, the group's deputy Asia director. "This whole process is being drawn out unnecessarily to maximize the Burmese government's leverage with the international community."
The last release took place in September, a week before Thein Sein visited New York for the U.N. General Assembly.
Thein Sein's government has spearheaded a major transition toward democracy, easing harsh media censorship, signing cease-fire deals with armed rebel groups, and opening the country more to Western investment.
But rights groups say Thein Sein has not yet consolidated the political and economic reforms. The military is still dominant and is implicated in rights abuses. It has failed to prevent outbreaks of communal violence in the west of the country that have left scores dead.