Barack Obama will become the first US president to visit the former pariah state when he touches down on Monday.
Ahead of the historic trip, the Burmese government has released hundreds of prisoners in an apparent gesture of goodwill.
Initially the announcement was welcomed but some rights groups have now criticised the move, saying there were no political prisoners among those released.
Burma Campaign UK Director, Mark Farmaner, told Radio Australia's Asia Pacific that while 700 to 800 political prisoners have been released by Burma in the last year, others are still being arrested and jailed.
These include farmers in northern Kachin state who have been accused of being in the Kachin Independence Army, and Rohingya political and community leaders.
Mr Farmaner says it is a 'drip drip' method of tactical political prisoner releases.
"We've said to the government of Burma, we need now to establish a proper Commission in Burma, which has international support, with UN expertise, which can properly assess how many political prisoners are in Burma's jails and ensure that they are released," he said.
The US says it will continue to push for further reforms when the president holds talks with senior officials.
But Mr Farmaner says after lifting most sanctions against Burma, President Obama is fast running out of leverage.
"He's thrown away the sticks, now he's given away almost all the carrots," Mr Farmaner said.
"So what leverage has he got left, to actually make sure that Thein Sein really delivers on releasing all the political prisoners, ending the violence against ethnic minorities and introducing real democratic reforms?"