A university professor has been hacked to death in Bangladesh, in an attack police say is similar to killings of secular bloggers and atheists by suspected Islamist extremists.

AFM Rezaul Karim Siddique, 58, was a professor of English at Rajshahi University in the country's north-west.

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Siddique is the fourth professor at the university to be have been killed in the past 12 years. It is not clear why they have been targeted and no culprits have been punished.

Earlier this month, a Bangladeshi law student who had expressed secular views online died when he was hacked with machetes and then shot in Dhaka.

The four bloggers killed last year had all appeared on a list of 84 "atheist bloggers" drawn up by Islamic groups in 2013 and widely circulated.

There have also been attacks on members of religious minorities including Shia, Sufi and Ahmadi Muslims, Christians and Hindus.

Several men are under arrest for last year's killings, including some attached to a hardline group called the Ansarullah Bangla Team.

Muslim-majority Bangladesh is officially secular but critics say the government has failed to properly address the attacks.

Last week, Prime Minister Sheikh Hasina issued a stern warning to anyone who criticised religion: "I don't consider such writings as freethinking but filthy words. Why would anyone write such words? It's not at all acceptable if anyone writes against our prophet or other religions."