Five Islamist extremists were sentenced to death on Tuesday over the brutal murder of a Bangladeshi-American writer and rights activist six years ago.

Avijit Roy, a prolific blogger and the author of 10 books including the best-selling "Biswasher Virus" ("Virus of Faith"), was hacked to death outside Bangladesh's largest book fair by machete-wielding extremists in February 2015.

The murder, part of a reign of terror by extremists at the time, enraged the Muslim-majority nation's secular activists, who staged days of protests.

The judge at Dhaka's Special Anti-Terrorism Tribunal found six people guilty, sentencing five to death and one to life in prison, prosecutor Golam Sarwar Zakir told AFP.

Two of them were tried in absentia, including sacked army officer Syed Ziaul Haque, who was accused of leading the group that carried out the attack -- known as Ansarullah Bangla Team, or Ansar al Islam.

A defence lawyer said they would appeal the verdict at a higher court.

Roy was born in Bangladesh in 1972 and moved to the United States in 2006 from where he continued to criticise the government for the jailing of atheist bloggers.

His wife Rafida Ahmed Bonya, who was injured in the attack and lives in the US, said the verdict would not bring her peace.

-In six years, not one person investigating the case in Bangladesh reached out to me -- though I am a direct witness and victim of the attack- she wrote on Facebook.

-Simply prosecuting a few foot-soldiers -- and ignoring the rise and roots of extremism -- does not mean justice for Avi's death.-

The verdict comes less than a week after eight Islamist extremists were sentenced to death for the murder of a publisher who brought out books by secular writers, including two by Roy.

The attacks were part of a wave of violence between 2013 and 2016 targeting secular activists, bloggers and atheist writers at a time of heightened political tensions.

Several top Islamist political party leaders were hanged over the violence under the government of Prime Minister Sheikh Hasina.

The Bangladesh government has since set up two major anti-terrorism police units to crack down on Islamist extremists.

More than 100 suspects have been killed in anti-terror raids and hundreds detained. Around half a dozen Islamist militant outfits have been banned.

Bangladesh cricket star Shakib Al Hasan has become the latest target of radicals, and had to be given an armed bodyguard after he was threatened for attending a Hindu ceremony in India.