DHAKA: A helicopter that was supposed to be taking guests to a wedding party landed at a maximum security jail outside Bangladesh's capital Dhaka by mistake on Thursday, the country's prisons chief said.

Armed guards surrounded the helicopter as soon as it touched down on the prison grounds, media reported, as the facility remains on alert after an attempt to free a top Islamist extremist as he was being transported there in March.

The five wedding guests were released after the private aviation company promised officers it would not happen again, said Brigadier General Syed Iftekhar Uddin.

"It landed in the prison by mistake. Meghna Aviation has said sorry. They said it happened due to poor visibility," Uddin told AFP.

But he added police were "concerned" following reports that Islamist extremists could be plotting to free fellow militants.

Bangladesh has detained scores of extremists in recent months as part of a massive nationwide crackdown that followed a wave of deadly attacks.

In April, top Islamist extremist leader Mufti Abdul Hannan was hanged at the same prison after he was convicted of a grenade attack on the British ambassador in 2004 and an array of blasts that left scores killed in the 2000s.

Before the execution, militants made a daring attempt to free Hannan as he was being taken to the jail in a prison van.