Bangladesh police escort Minhajul Abedin Russel (right), Russel Chowdhury (second right), Shakhawat Hossain (second left) and Tamjid Ahmed Rubel (left) after they were arrested over the murder of Italian citizen Cesare Tavella in Dhaka on Monday.

AFP/Dhaka

Bangladesh made its first arrests over an Italian aid worker's murder as authorities insisted on Monday the killing was a plot by the government's opponents to trigger anarchy and not the work of the Islamic State group.

Police paraded four people detained over last month's killing of 50-year-old Cesare Tavella along with a motorbike said to have been used as a getaway vehicle in the shooting late last month in Dhaka.

The killing near the capital's diplomatic zone was the first of a series of attacks to be claimed by Islamic State (IS) and was followed days later by the gunning down of a Japanese farmer in northern Bangladesh.

A weekend bombing of the main Shia shrine in Dhaka, which killed one person and wounded dozens more, has further heightened the fears of minorities living in the mainly Muslim but officially secular nation.

But although that attack was also claimed by IS, the government responded on Sunday by denying the extremist group was active in Bangladesh and instead rounded up dozens of Prime Minister Sheikh Hasina's opponents.

Speaking after the four suspects were brought before the media, Dhaka police said they had all admitted killing Tavella on "the orders of a so-called big brother who offered them money".

"They carried out the murder to embarrass the government, to put the government under pressure and to create anarchy," Dhaka police commissioner Asaduzzaman Mia told reporters.

"So far what they've admitted to us is that they were not specifically targeting Cesare Tavella but their aim [was to] kill any white-skinned foreigner.

"Their big brother wanted to prove that Bangladesh is not safe for foreigners and if they could prove that, then it would put pressure on the government."

Police described two of the suspects as drug addicts who have previous criminal convictions while another was a drug-dealer and the fourth was "a cold-blooded killer".

Mystery 'big brother'  

They did not elaborate on the identity of the "big brother" said to have ordered the killing of Tavella who worked for a faith-based Dutch charity.

But Mia insisted it could not have been carried out by IS or another extremist religious group behind the recent killings of atheist bloggers.

"There is no militant connection with this killing. This is entirely a local group which wants to make political capital," he said.

His comments echoed those of Home Minister Asaduzzaman Khan Kamal who told reporters after the shrine blast that "the IS organisation does not exist in Bangladesh".

Bangladesh prides itself on being a mainly moderate Muslim nation but the gruesome killings of a series of atheist bloggers this year have rocked the nation and sparked a crackdown on local hardline Islamist groups.

After the foreigners' murder, international schools closed temporarily and embassies restricted their diplomats' movements, while Australia's cricket team cancelled a planned tour over security concerns.

As well as the attacks claimed by religious hardliners, Bangladesh has been plagued by deadly political violence blamed on followers of the main opposition Bangladesh Nationalist Party which boycotted elections in 2014.

Hasina and her allies have consistently blamed the BNP and its main Islamist ally Jamaat-e-Islami for both the unrest and the attacks on foreigners and religious minorities.

After the attack on the Shia shrine, a former BNP lawmaker in Dhaka and several senior officials from both parties were detained.

Although police said the arrests were related to previous unrest and not linked to Saturday's blast, the BNP said the government was clearly using the heightened state of anxiety as a pretext to crack down on opponents.