A coalition of human rights groups called on the United States on Monday to grant temporary visas to secular writers from Bangladesh after a series of bloody attacks by Islamist militants.

Five bloggers and a publisher were killed by Islamists in Bangladesh this year, including an American citizen of Bangladeshi origin. The rights groups, led by the PEN American Center, which advocates freedom of expression, said at least four others had been attacked.

In a letter to US Secretary of State John Kerry, the eight groups - including Freedom House, Human Rights Watch and Reporters Without Borders - said the writers were in "urgent danger."

Karin Deutsch Karlekar, PEN's director of free expression programs, said dozens of Bangladeshi writers were living in hiding and seeking "protections their own government is unwilling or unable to provide."

"Bloggers and writers in Bangladesh have nowhere left to turn, as they face both death threats by extremist groups and fear of arrest on charges of blasphemy by government officials seeking to appease religious authorities," she said in a statement.

Suzanne Nossel, executive director of PEN America, said the writers were "terrified," and should be allowed to enter the United States under a system known as humanitarian parole, which grants visas to individuals at risk who would otherwise be inadmissible.

"Having championed global efforts to defend Internet freedom and fend off threats to religious liberty, the United States should take the lead to save the lives of these bloggers who face the very real risk of being murdered for the crime of expressing their views online," Nossel said.