Bangladeshi police stand guard in front of the high court in Dhaka. File/AFP

Avijit Roy, an engineer of Bangladeshi origin, was hacked to death by machete-wielding assailants in February 2015 while returning home with his wife from a Dhaka book fair. His wife, blogger Rafida Bonya Ahmed, suffered head injuries and lost a thumb in the attack.

Five members of an militant group were sentenced to death last year, while one was jailed for life.

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Two of those on death penalty escaped on Sunday, after bikers sprayed chemical on the police before snatching away the convicts, police said.

"A massive manhunt has been launched to capture them and their helpers," Home Minister Asaduzzaman Khan told reporters, adding that border security officials had been put on alert to stop the men fleeing the country.

Police has also announced an award of two million taka (about $19,350) for tracing the convicts.

The convicted men belong to the Al Qaeda-inspired domestic militant group Ansar Ullah Bangla Team and police say the group was behind the murders of more than a dozen secular activists and bloggers.

Muslim-majority Bangladesh saw a string of deadly attacks between 2013 and 2016 targeting bloggers, secular activists and religious minorities, claimed by Daesh or Al Qaeda-aligned groups.

Reuters