Tokyo: The families of seven Japanese killed by Islamist militants in Bangladesh prepared on Sunday to retrieve the remains of their loved ones, in shock at the deaths of the development-aid workers caught up in one of the most brazen attacks in the South Asian nation's history.

"I'm really embittered," Fumie Okamura, mother of victim Makoto Okamura, 32, told Reuters before the families boarded a government flight to Bangladesh capital Dhaka, where 20 people were killed in an upmarket restaurant on Friday.

Foreign Minister Fumio Kishida met the families before they left and promised them the government's utmost support.

Makoto Okamura, an employee of construction consulting firm Almec Corp, was working on an urban-transport project commissioned by the Japan International Cooperation Agency, which manages the Japanese government's overseas aid projects.

"My son wanted to help developing countries," his mother told Reuters by phone from her home on the eastern outskirts of Tokyo. "He wanted to work in the field of urban-transport engineering, which was his dream since when he was in junior high school.

"I'm a doting mother, but I'm proud to say he was the perfect son."

Japan has been heavily involved in Bangladesh infrastructure projects since Prime Minister Shinzo Abe pledged in 2014 to provide suport for what is one of the world's poorest countries.

"It was an unpardonable act of terrorism," Abe told reporters after a National Security Council meeting to discuss Friday's attack.

Italians and Americans were also among the victims of the attack, which could damage the confidence of the expatriate community in Bangladesh, many of whom work for multinationals in the country's $26 billion garment industry.

Makoto Okamura's boss, Tamaoki Watanabe, was rescued after being shot in the attack. Six other Japanese men and women aged from their twenties to eighties were killed, Chief Cabinet Secretary Yoshihide Suga told reporters.

Among the victims were engineer Koyo Ogasawara, of Katahira & Engineers International, and Nobuhiro Kurosaki, of Oriental Consultants Co, domestic media reported. The government would not disclose the names of the victims.