Dhaka (AFP) - Unidentified attackers hacked to death a university professor in Bangladesh on Saturday, police said, adding that the assault bore the hallmarks of previous killings by Islamist militants of secular and atheist activists.

Police said English professor Rezaul Karim Siddique, 58, was hacked from behind with machetes as he walked to the bus station from his home in the country's northwestern city of Rajshahi, where he taught at the city's public university.

"His neck was hacked at least three times and was 70-80 percent slit. By examining the nature of the attack, we suspect that it was carried out by extremist groups," Rajshahi Metropolitan Police commissioner Mohammad Shamsuddin told AFP.

Shamsuddin said police had not yet named any suspects but added that the pattern of the attack fitted with previous killings by Islamist militants.

Nahidul Islam, a deputy commissioner of police, told AFP that Siddique was involved in cultural programmes, including music, and set up a music school at Bagmara, a former bastion of an outlawed Islamist group, Jamayetul Mujahideen Bangladesh (JMB).

"The attack is similar to the ones carried out on (atheist) bloggers in the recent past," Islam said, adding nobody had been arrested yet.

Homegrown Islamist militants have been blamed for a number of murders of secular bloggers and online activists since 2013, the most recent being in the capital Dhaka early this month.

Police said that in each of the attacks unidentified assailants hacked the victim to death with machetes or cleavers.

Eight members of banned Islamist group Ansarullah Bangla Team, including a top cleric who is said to have founded the group, were convicted late last year for the murder of atheist blogger Ahmed Rajib Haider in February 2013.

- 'Gruesome pattern' -

Sakhawat Hossain, a fellow English professor from the university and a friend, said the slain teacher played the tanpura, a musical instrument popular in South Asia, and wrote poems and short stories.

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"He used to lead a cultural group called Komol Gandhar and edit a bi-annual literary magazine with the same name. But he never wrote or spoke against religion in public," Hossain told AFP.

Hundreds of students of Rajshahi University staged impromptu protests, marching on the campus in batches and shouting slogans, demanding the arrest of the killers, local police chief Humayun Kabir told AFP.

"The students were shocked at the latest brutal killing of their teachers. Some 500 of them shouted slogans and joined the marches calling for protection of all teachers and exemplary punishment for the killers," Mostafiz Mishu, a student who witnessed the protests, told AFP.

Police said Siddique was the fourth professor from Rajshahi University to have been murdered. In February, a court handed down life sentences to two Islamist militants for the murder of another professor, Mohammad Yunus.

The recent killings have sparked outrage at home and abroad, with international rights groups demanding that the secular government protect freedom of speech in the Muslim-majority country.

Champa Patel, Amnesty International's South Asia director, condemned the latest killing as "inexcusable", saying it was part of a "gruesome pattern".

"The authorities must do more to put an end to these killings. Not a single person has been brought to justice for the attacks over the past year," Patel said.

Ansar al-Islam, a Bangladesh branch of Al-Qaeda in the Indian Subcontinent, this month claimed responsibility for the murder of 26-year-old Nazimuddin Samad, a law student who was killed on the streets of Dhaka, according to US monitoring group SITE.

Police, however, blamed the Ansarullah for the murder.

Bangladesh authorities have consistently denied that international Islamist networks such as Al-Qaeda or the Islamic State group, which recently claimed responsibility for the murders of minorities and foreigners, are active in the country.

A long-running political crisis in the majority Sunni Muslim but officially secular country has radicalised opponents of the government and analysts say Islamist extremists pose a growing danger.