A free speech group launched a blistering attack on Sweden on Wednesday after it denied a travel visa to a secular Bangladeshi blogger who was then hacked to death by suspected Islamists.

Swedish PEN, which defends freedom of expression, slammed the refusal as "horrific" and said it would demand an explanation from the authorities, claiming Ananta Bijoy Das would be still alive if the visa had been granted.

"We want to know where the directive came from," Swedish PEN official Elnaz Baghlanian told AFP.

A masked gang wielding machetes hacked Das, 33, to death Tuesday in broad daylight as he headed to work at a local bank in Sylhet in northeastern Bangladesh, the third such deadly attack by suspected Islamists since February.

Another blogger, Washiqur Rahman, was hacked to death in March and writer Avijit Roy was killed in February.

Bangladeshi police would not comment on the motive for Das' killing, but fellow writers said he had been on a hitlist drawn up by militants who were behind Roy's killing.

Swedish PEN had invited Das to Stockholm to speak on May 3, World Press Freedom Day, about the deteriorating situation in Bangladesh for journalists and writers.

But the Swedish embassy in Dhaka had refused him a travel visa, according to PEN.

"You belong to a category of applicants where there is always a risk involved when granting a visa that you will not leave (the) Schengen area after the visit. Furthermore, the purpose of your trip is not urgent enough to grant you (a) visa," read an excerpt of his rejection letter from the Swedish embassy in Dhaka, published on PEN's website.

Swedish foreign ministry spokesman Kent Oberg confirmed the embassy had rejected his application, but provided no details.

"It's horrific that the embassy is more concerned that he will stay in the country than that he will be murdered," Baghlanian said.

"He would have been alive if he had been here, he was supposed to be here for two weeks," she said.

- Death threats -

Sweden has one of the European Union's most generous asylum policies, granting asylum to more than 70 percent of all refugees.

Baghlanian said she did not know whether Das had considered seeking asylum in Sweden.

"We don't know anything about that. And now we'll never know," she said.

Friends said Das was an editor of a quarterly magazine called Jukti (Logic) and headed the Sylhet-based Science and Rationalist Council.

They said he had received threats from Islamists after he regularly blogged for a website called Mukto-Mona, founded and moderated by Roy, in which he criticised aspects of Islam and Hinduism.

On Wednesday, secular activists marched through Sylhet to demand justice for Das, and accused the government of failing to protect free thinkers.

The recent deaths have sparked international condemnation, with Washington on Tuesday calling on Dhaka to bring the perpetrators of the latest killing to justice.

New York-based Human Rights Watch said Das' murder was part of an "alarming" trend of violent intolerance of religious freedom and speech in Bangladesh.

"This pattern of vicious attacks on secular and atheist writers not only silences the victims but also sends a chilling message to all in Bangladesh who espouse independent views on religious issues," said Brad Adams, Asia director at HRW.