Police in Bangladesh have arrested four suspects in the killing of an Italian aid worker last month and said that the alleged gunmen had confessed to being hired to attack "a white man".

Cesare Tavella was shot by motorcycle-riding assailants on September 28 while jogging in the diplomatic quarter of Dhaka, Bangladesh's capital.

Five days later, a Japanese citizen was killed in a similar attack in northern Bangladesh.

Locals groups blamed

Responsibility for the attack on Tavella was quickly claimed by a group affiliating itself with the Islamic State of Iraq and the Levant (ISIL) group, but Bangladesh's government rejected the claim and said there was no evidence that ISIL had any presence in the country.

It also rejected ISIL's claims of killing the Japanese man and for the bombing of a Shia procession over the weekend that left a teenage boy dead and more than 100 people injured.

Bangladesh has instead accused domestic groups along with Islamist political parties - specifically the opposition Bangladesh Nationalist Party and its main ally, Jamaat-e-Islami - of orchestrating the violence to destabilise the already fractious nation.

On Monday, Dhaka Metropolitan Police Commissioner Asaduzzaman Mia said the latest arrests supported that theory.

'A big brother'

The suspects, including known professional killers identified from closed-circuit television footage from the attack site, allegedly told investigators they had been hired by "a big brother" to kill Tavella in order to "create chaos" in the country, Mia said.

"We have identified that so-called big brother. When we are able to arrest that man, it will be clear who used them," the police commissioner said.

Mia said that the attackers targeted Tavella simply because he was Caucasian, and that the suspects were promised payment for the attack.

"Tavella was not their specific target, he just became a victim being a white [man]," Mia said. "The killers asked whether he [the man who hired them] had any enmity with that foreigner; he replied, 'No, you have to kill a white man.'"

The killers also said they had received half of the payment promised for the attack, according to Mia. "We have interrogated them separately. We need to interrogate them more, with permission from court," he said.

The suspects were not involved in the assault on the Japanese man, Mia said. "That's a separate group," he said, without giving any details or saying whether authorities had any leads.

The arrests were considered a major breakthrough for Bangladesh, which has been struggling to stem a rising tide of violence, including the murders of four atheist bloggers claimed by armed groups this year.