A group of prisoners at a jail in southern Taiwan took three staff hostage Wednesday and demanded that they be released, complaining about their lengthy sentences and claiming their trials were unfair, officials said.

The six inmates broke into a weapons storage room in the Kaohsiung city jail at about 4:00 pm (0800 GMT), obtained four rifles and six handguns, and took three staff hostage.

They later agreed to release them in exchange for the prison chief and another senior staffer, justice ministry officials said.

The six, who are in jail for a variety of crimes including murder, robbery and drugs, claim they were unfairly tried and are demanding their release from prison.

“Negotiations with the kidnappers are underway. They demanded they leave the prison from a side door,” deputy justice minister Chen Ming-tang said.

At 11:00 pm the standoff was ongoing.

Lee Rong-tsung, a former councillor at the Kaohsiung City Council who reportedly knew one of the inmates, said police had asked him to speak with the inmates to hear their grievances.

“They said they were unhappy about their unfair trials, and they complained that their jail terms were too long,” Lee told a television station.

The leader of the six was identified by police as Cheng Li-teh, a senior member of a notorious triad group known as the Bamboo Union Gang.

Cheng, sentenced 28 years for murder, has been behind bars since August 2012, according to the justice ministry.

Cheng said it was “unfair” that Taiwan’s former president Chen Shui-bian — who was sentenced to 20 years for graft — was given parole for medical treatment last month, according to the United Daily News website.

Later Wu Hsien-chang, the chief of the ministry’s correction agency, read a statement from Cheng.

“Chen Shui-bian was an inmate too, but why he could be paroled and described as a political prisoner?” it said.

Cheng also complained about the small daily allowance inmates receive, his trial and said sentence reductions should also be applicable to prisoners on felony charges.

Television images showed police armed with rifles and protective gear surrounding the prison and a senior Kaohsiung police officer confirmed that more than 250 police have been mobilised.

Chen said the kidnappers had fired shots into air, apparently as a warning, but no one had been hurt.