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 has died while he's been in prison.

Sharmina Parveen told AFP her family had also been ‘ostracised’ since Hasnat Karim, 48, was named a suspect in the Dhaka cafe attack which left 20 hostages dead four months ago.

Sharmina, 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 Sharmina's family.

However, Karim was detained by 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.

Sharmina insists her husband, a British citizen of Bangladeshi origin, was being 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 order 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.

Bangladesh authorities have blamed a local extremist group for the cafe attack, rejecting claims by the Islamic State organisation that it was behind the carnage.

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

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

Sharmina said Karim's father, who had been receiving kidney dialysis, had passed away on Monday.

‘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 Tuesday.

‘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 schoolteacher and that her family had been shunned since 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,’ Sharmina told AFP.

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