Indian among 20 killed in Bangladesh's worst hostage crisis.

​Dhaka/New Delhi: Islamic State terrorists slaughtered 20 civilians, mostly foreigners, including an Indian, during an overnight siege at an upscale cafe in the Bangladesh capital before security forces rescued 13 hostages and killed six gunmen on Saturday morning, ending the nation's worst hostage crisis, being termed as Bangladesh's '7/16'.

Bangladesh Prime Minister Sheikh Hasina has called a two-day state mourning for the victims, who included nine Italians, seven Japanese, three Bangladeshis and an Indian teenager, at the Holey Artisan café in the diplomatic area of Gulshan in Dhaka.

One of the gunmen, injured in the shootout, was captured, while 13 hostages were rescued at the end of the 12-hour siege. The Islamic State has claimed responsibility for the attack.

Indian President Pranab Mukherjee, Prime Minister Narendra Modi and West Bengal Chief Minister Mamata Banerjee have condemned the attack.

Modi called up Sheikh Hasina and condemned the "despicable attack" and said that India "stands firmly with our sisters and brothers of Bangladesh" in this hour of grief.

Indian External Affairs Minister Sushma Swaraj confirmed the killing of 19-year-old Tarishi Jain of Firozabad in Uttar Pradesh.

"Tarishi was 19 years old. She passed out from American School Dhaka. Presently, she was a student at Berkeley," Sushma posted.

In Dhaka, Brigadier General Nayeem Ashfaq Chowdhury, director of military operations, told reporters that "most of the victims were killed brutally with sharp weapons".

The attack began at 8.45 p.m. when around 20-22 guests were at the Holey Artisan Bakery downstairs and the O'Kitchen Restaurant upstairs, a cafe popular with foreigners.

The gunmen, shouting "Allahu Akbar" raided the cafe and took hostages, and slaughtered those who were unable to recite the Quran, said rescued hostages.

Two policemen were also killed in the initial hours of the rescue mission. Later, the area was sealed off, and following directives from Prime Minister Hasina, the armed forces along with the Rapid Action Battalion and police launched an assault, codenamed 'Operation Thunderbolt', in the morning. The siege ended at 8.30 a.m.

Prime Minister Hasina, condemning the "extremely heinous act", vowed to root out terrorism from the country, which has seen a spate of deadly attacks by Islamic State and Al Qaeda-linked militants on progressive academics, writers, activists and religious minorities in the Muslim majority country.

"We'll establish Bangladesh as a peaceful state…No conspiracy can hinder our advancement," she said in a nationally televised address.

"What kind of Muslims are these people? They don`t have any religion. People must resist these terrorists. My government is determined to root out terrorism and militancy from Bangladesh," she said.

Officials said the 13 rescued hostages included a Japanese and two Sri Lankans.

The Bangladeshi branch of the Islamic State claimed the attack through its mouthpiece, the Amaq news agency, saying 24 people "of different nationalities" were killed and 40 others were injured.

The Daily Star reported that hostages were made to recite verses from the Quran and those who could were not harmed.

"The others were tortured by the gunmen," Rezaul Karim, father of one of the hostages Hasnat Karim, told the daily. Karim had gone to celebrate his 13-year-old daughter's birthday along with his wife and son Rayan, 8. The family was rescued early in the morning.

The attack came as a Hindu priest was killed in Bangladesh on Friday, while another Hindu priest was grievously injured in a murderous attack on Saturday.

Neighbouring India called for quick action to adopt the long-stalled Comprehensive Convention on International Terrorism (CCIT) and expressed disappointment that the UN General Assembly failed to push for its early adoption.

"The perpetrators of terrorist attacks as well as the states that support and sponsor or provide safe havens to terrorists or terrorist groups must be made accountable," India's Permanent Representative Syed Akbaruddin said, speaking at the General Assembly.

The attack has also been condemned by other countries, including Pakistan and Malaysia, while the European Union has also voiced condemnation.