Burma began releasing 452 prisoners under an amnesty on Thursday, but pressure groups and relatives say they have not yet heard of any political detainees being among those freed.
Activists said it was very disappointing and have accused the government of playing games.
The government announced the move days before a visit to Burma by US President Barack Obama.
Since the end of military rule in 2011, hundreds of dissidents have been freed.
But it is estimated that at least 300 are still in jail.
Correspondents point out that many political detainees are in remote areas where communications are difficult, so the full extent of the releases may not be known for some time.
Past releases have included common criminals and prisoners of conscience.
The Assistance Association for Political Prisoners (AAPP), a Thailand-based campaign group, said that Thursday's releases were "the worst amnesty to date".
Likewise, the New York-based Human Rights Watch (HRW) group and members of opposition leader Aung San Suu Kyi's National League for Democracy (NLD) cast doubt over the latest amnesty.
"It seems there is a new game in Burma, which is the political prisoners game," said HRW deputy Asia director Phil Robertson.
He described the non-release of political prisoners as "cynical" and lacking in transparency.
Senior NLD representative Nie Nie accused the government of misinformation ahead of President Obama's visit.
"They are lying to Obama about releasing political prisoners," he said. "They did not release political prisoners."
The government earlier said that the releases were intended to "help promote goodwill and the bilateral relationship".
The move is the latest in a series of prisoner releases under the government of President Thein Sein.
Mr Obama's visit, the first by a serving US president, is intended to encourage continued government reform.
The last amnesty in September saw the release of more than 500 prisoners, with the opposition saying that at least 58 political detainees were freed.
The nominally-civilian government led by Thein Sein has undertaken a series of reforms since the end of military rule in elections in November 2010.
These include freeing hundreds of prisoners, including political detainees and introducing more press freedom. Western sanctions against Burma have been loosened in response.
The NLD rejoined the political process after boycotting the November 2010 polls and overwhelmingly swept parliamentary by-elections earlier this year.
However, critics question Mr Obama's decision to make Burma one of his first destinations after re-election, saying it is too soon to reward the government for reforms which have just started, the BBC's Jonathan Head in Bangkok says.