Opposition leader Aung San Suu Kyi and her party the National League for Democracy will participate in upcoming byelections in Burma this April. A presidential aide claims the NLD may one day rule the government. This could be a historic moment, but only if the country's remaining political prisoners are free and can participate.

Currently in Burma, William Hague, the British foreign secretary, has expressed "hope to see the release of all remaining political prisoners". But while the Burmese government has spoken about releasing them, it continues to disappoint by holding more than a thousand political prisoners behind bars.

An October 2010 amnesty saw only about 220 released, including famed comedian Zargana. One year ago, Zargana languished in a remote prison for criticising the government's response to Cyclone Nargis. A year later, he's visiting Thailand and Cambodia to promote his new film festival and is quoted on the front page of the English-language edition of the Myanmar Times.

But a much-anticipated 2 January presidential clemency order was a disappointment. It reduced prison sentences for common criminals, resulting in the release of only about a dozen political prisoners. For those sentenced to lengthy prison terms like monk leader U Gambira, activist Min Ko Naing and members of the 88 Generation Students group, their sentences were simply cut to 30 years.

Exactly how many political prisoners remain behind bars has become a major bone of contention. The government claims that before the October releases there were 526 "national security" detainees, now leaving only 300. But this leaves out many known prisoners. Hague's challenge is to persuade the government to publicly account for all remaining political prisoners.

Run for almost 50 years by a military junta, Burma has long been notorious for holding political prisoners. Open opposition to the government resulted in long, swift sentences under cruel conditions. Perhaps only 300 people are imprisoned under specific charges, but it's still unclear which laws the Burmese government is talking about. And many more remain jailed on trumped-up politically motivated criminal charges.

Journalist Hla Hla Win, for instance, was arrested in 2009 while interviewing monks. She was sentenced to seven years for using an unregistered motorbike, then another 20 for uploading data to the internet that was "damaging to the security of the military regime". Monks participating in the 2007 protests were charged with insulting religion, and others have been charged with illegally holding foreign currency, possessing electronic equipment without a license, and immigration violations.

The Thai-based Assistance Association of Political Prisoners of Burma is composed of former Burmese political prisoners who have tracked individual cases for more than a decade. It estimates there are more than 1,500 political prisoners. The US state department has a list of around 1,100, but does not rule out that the true number is higher. The NLD has produced a partial list of 591 political prisoners, based on information gathered from NLD lawyers, social workers and local party officials.

Given the closed nature of Burma's legal system, the lack of a free press, and unsophisticated communications in one of Asia's poorest countries – particularly in remote ethnic areas affected by conflict – each of these lists probably omits significant numbers of people being held for peaceful expression of their political views. For years Burma's prisons have been off-limits to any independent monitoring mechanism.

Hague should call on the Burmese government to allow an independent international body to identify each prisoner and determine whether the person is imprisoned on political grounds. While some have said the new National Human Rights Commission could perform this role, it has yet to establish its independence and lacks capacity and experience. Hague should make clear that any new detentions on political grounds will call into question the government's commitment to change.

After his release, Zargana summed up the feelings of many in Burma when he said: "I am not pleased to see what they are doing. They are doing it bit by bit. We are like the hostages captured by the Somali pirates. It's like how much ransom money can you pay to secure the release of these hostages?"

The Burmese government needs to show the world that it sees imprisoned activists as part of the country's future, not hostages to be parleyed as evidence of the sincerity of their touted reforms. The full and unconditional release of all political prisoners is an essential step toward an inclusive political process.

• Follow Comment is free on Twitter @commentisfree