Bangladesh has ordered a probe into a sharp spike in secret killings and disappearances in the country, many of which are allegedly linked to the security services, the home minister said Wednesday.

"I've already told (law enforcement agencies) to investigate incidents of disappearances or secret killings. They are already working on this," Sahara Khatun told AFP, after meeting with the country's police chiefs.

Her comments came a day after the head of the state-run national human rights commission said the government had to prove that law-enforcement agencies were not linked to the rise in disappearances.

According to the local human rights group Odhikar, at least 40 people -- many of them opposition political activists -- disappeared between January last year and November 2011.

This is only a "fraction" of the real number, Odhikar secretary Adilur Rahman Khan said, adding that the majority of the cases were linked to law enforcement agencies, including the elite paramilitary Rapid Action Battalion.

"What is alarming is that the families of the victims have told us members of Rapid Action Battalion, detective branch police or police took their relatives. And they never returned," he said.

All law-enforcing agencies have rejected claims they are involved in cases of disappearances.

On Wednesday police found dead bodies of three people in a river outside the capital. They showed head wounds, had their hands tied and had been deliberately sunk with cement bags, police inspector Mizanur Rahman told AFP.

The incident came two days after the recovery of two more dead bodies, including that of an activist of the main opposition party, in the same river, he added.

Sumi Das, whose husband disappeared from the capital Dhaka four months ago, said Tapan Das was taken away by a group of people who identified themselves as the members of detective branch police. Das never returned, she told AFP.