Strong condemnations poured in following the brutal murder of secular blogger Niladri Chattopadhyay Niloy by suspected Islamist extremists yesterday.

Criticising the killing, the United Nations, the Amnesty International and the Committee to Protect Journalists (CPJ) expressed deep concerns over threats to free speech.

"The violent killing of another critical voice in Bangladesh shows that serious threats to freedom of expression persist in the country," the UN special rapporteurs on freedom of expression, David Kaye, and on extrajudicial executions, Christof Heyns, said in a statement.

They called for a prompt and thorough investigation into the recent murders of bloggers critical of extremism.

"It is vital to ensure the identification of those responsible for this and the previous horrendous crimes, as well as those who may have masterminded the attacks.

"The organised targeting of critical voices aims at promoting a culture of silence and fear, and affects the society as a whole."

The special rapporteurs said the Bangladeshi authorities must not only continue to strongly condemn these horrendous acts against freedom of expression, but should also ensure that their words are followed by more effective efforts to ensure greater accountability and prevent this kind of violence.

They urged the government to consider specific initiatives to prevent the recurrence of attacks against writers and activists. This should include not only the provision of particular physical protections to those who are potential targets for violence, but also open public debate challenging extremist views of all kinds.

Meanwhile, the UN resident coordinator in Bangladesh called upon the authorities to investigate and bring the perpetrators of this killing and all the previous ones to justice in a speedy manner.

"The police need to take concrete measures to protect citizens. Moreover, strong condemnation of these incidents from the all levels of society will be important to prevent further murders," Robert D Watkins said.

The Amnesty International (AI) said the Bangladesh government must send a strong message that killings aimed at silencing dissenting voices are despicable and will not be tolerated.

"This spate of savage killings must end here. There is little doubt that these especially brutal killings are designed to sow fear and to have a chilling effect on free speech. This is unacceptable," said David Griffiths, AI's South Asia research director.

"The price for holding opinions and expressing them freely must not be death. The Bangladeshi authorities now have an urgent duty to make clear that no more attacks like this will be tolerated."

Griffiths said that thorough, effective, independent and impartial investigations must be carried out promptly to ensure all those responsible are brought to justice in fair trials without recourse to the death penalty.

"How many more bloggers must be murdered before the government of Prime Minister Sheikh Hasina acts decisively to stem the violence and impunity?" Asia Program Research Associate Sumit Galhotra of the New York-based CPJ said, following Niladri's killing.

"We call on the Hasina government to take urgent steps to bring the perpetrators of this barbaric murder to justice and protect all journalists under threat," he added.

Earlier in February this year, secular blogger Avijit Roy was killed and his wife seriously injured in a similar attack. A month later, another blogger named Oyasiqur Rahman was killed in the capital. Then in May, Ananta Bijoy Das, who wrote for a website promoting secularism and religious tolerance, was hacked to death in Sylhet.