Police stand guard at the site where a Japanese citizen was shot dead in Rangpur last month.

AFP/Dhaka

Bangladesh security forces on Friday arrested three suspects wanted over the murder of a Japanese farmer who was shot dead last month in an attack claimed by the Islamic State group, an official said.

The Rapid Action Battalion (RAB) said the three youths had been detained early Friday in a hideout 210 kilometres from Rangpur where 66-year-old Hoshi Kunio was gunned down in a rickshaw last month.

After arresting them for an unrelated offence, the officers became suspicious and interrogated the men over Kunio's murder, leading the three to admit to their "involvement" in the killing, a spokesman for the elite unit told AFP.

"We have confirmed these men are wanted as fugitive criminals from Rangpur," said senior RAB commander Mobasshwer Rahim.

"Their appearances also match the description of the unknown assailants who shot Hoshi Kunio dead."

The suspects, named as Rajib Hossain, Kala Rubel and Kajal Chandra Barman, were found in the hideout armed with machetes and handguns, the force said.

"During primary questioning they've admitted about they were involved with the killing and since then they became fugitives," Rahim said.

"We've handed them over to police for further interrogation."

The arrests take the total number of people in custody to five after police last month detained an opposition party activist and a businessman in connection with the murder on a dirt track on the outskirts of the northern city.

The other two remain in custody although they have not been named as suspects.

Kunio's murder came just five days after an Italian aid worker, Cesare Tavella, was shot dead in similar fashion in the Bangladeshi capital Dhaka, an attack also claimed by the jihadist group IS.