DHAKA (AFP) - The wife of a British Bangladeshi man arrested over the South Asian country's deadliest attack says her husband is innocent and unaware that his father died while he has been in prison.

Ms Sharmina Parveen told Agence France-Presse her family had also been "ostracised" since Mr Hasnat Karim, 48, was named as a suspect in the Dhaka cafe attack, which left 20 hostages dead four months ago.

Ms Sharmina, Mr Karim and their two children were at the Holey Artisan Bakery in Bangladesh's capital when five gunmen stormed the cafe on July 1. Eighteen foreigners died.

Military commandos brought an end to the siege the next morning, killing all five attackers and freeing more than a dozen Bangladeshi hostages, including Ms Sharmina's family.

However, Mr Karim was detained by the authorities after video footage, which emerged hours after the hostages were shot and hacked to death, showed him strolling on the roof of the cafe with the attackers.

Ms Sharmina insists her husband, a British citizen of Bangladeshi origin, was used as a "human shield" by the hostage-takers.

"I was there in the cafe with him. He was taken to the rooftop at gunpoint. He only followed their orders to save his family... any responsible man would do the same," she said.

Bangladesh has been reeling from a wave of recent attacks, with targets including foreigners, rights activists and members of religious minorities.

The Bangladesh authorities have blamed a local Islamist extremist group for the cafe attack, rejecting claims by the Islamic State in Iraq and Syria (ISIS) organisation that it was behind the carnage.

Since the deadly assault, security forces have killed at least 40 Islamist militants, including a Bangladeshi-origin Canadian, who police described as the mastermind of the attack.

Mr Golam Mostafa, a lawyer for Mr Karim, who is a former university professor, has said there is no evidence his client was involved. Police have interrogated Mr Karim for several weeks, but are yet to formally charge him.

Ms Sharmina said Mr Karim's father, who had been receiving kidney dialysis, died on Monday (Oct 31).

"My husband doesn't know about his father's death. I'm very scared to give him the news in the jail as his cardiac condition is very weak," she said on Tuesday (Nov 1).

"The head of the family is gone. He was worried about his son till his last breath. It's been a very difficult phase," she added.

The 34-year-old said she was sent on "forced leave" from her job as a school teacher and that her family had been shunned since Mr Karim's arrest.

"My kids couldn't go to school for more than three months as they were facing social ostracisation. Other kids bullied my eight-year-old son saying that his father is a terrorist," Ms Sharmina told Agence France-Presse.

"My colleagues and friends stopped contacting me. The situation is unbearable," she said.