Burma is marking the 64th anniversary of its independence from Britain on Wednesday, amid criticism that the government's recent clemency program is not doing enough for political prisoners.

In a statement commemorating independence day, President Thein Sein praised the military's role in bringing about the recent democratic reforms, saying Burma is “marching towards a democratic nation of justice and liberty.”

He insisted the army will remain an essential pillar of the country.

Meanwhile, U.S. State Department spokesperson Victoria Nuland urged Burma to release all of its remaining political prisoners.

In a statement marking the anniversary, Nuland said President Thein Sein's recently enacted clemency program had fallen short of Washington's expectations. She said Washington remains committed to improving relations with Burma, but that normalizing ties will be difficult unless the government releases the estimated 1,000 political prisoners still in its custody.

On Tuesday, Burma began releasing hundreds of prisoners as part of the program, but rights activists were disappointed that very few prisoners of conscience were among those released.

Under the clemency, some death sentences will be commuted to life imprisonment. Prison terms above 30 years will be reduced to 30 years, and those between 20 and 30 years will be cut to 20 years. Shorter sentences will be cut by a quarter.

But many activists had hoped the program would provide amnesty rather than sentence reductions for those serving long sentences.

Activists also worry the clemency will not affect Burma's high-profile dissidents, many of whom are associated with opposition leader Aung San Suu Kyi's National League for Democracy.

In November, U.S. Secretary of State Hillary Clinton made a landmark visit to Burma, where she said the U.S. would consider lifting sanctions if Burma's new government continues to make political reforms.

For the past two decades, the U.S. has imposed sanctions on the former Burmese military government because of its harsh human rights abuses, including military operations against ethnic groups and the jailing of up to 2,000 political prisoners.

Burma's new military-dominated government has made a series of political reforms in the past year. Besides releasing some of its political prisoners, it also eased press restrictions and opened dialogue with some of its critics, including pro-democracy leader Aung San Suu Kyi.