 -- Twenty hostages were killed by militants who stormed an upscale restaurant in the Bangladeshi capital of Dhaka during a 10-hour siege, the Bangladeshi government announced Saturday morning.

U.S. citizens in the city were warned by the State Department to "remain vigilant" following the terror attack.

The breakdown of the victims' nationalities was unknown. But the restaurant, Holey Artisan Bakery, is located in Dhaka's diplomatic quarter and is popular with foreigners. Japan said seven of its citizens were unaccounted for.

At least six attackers were killed by Bangladeshi paramilitary troops who mounted rescue operations. One of the attackers was arrested.

The commandos also rescued 13 hostages.

Bangladesh Brig. Gen. Nayeem Ashfaq Chowdhury said troops recovered explosive devices and sharp weapons from the scene.

About 35 people were taken hostage, including about 20 foreigners, when militants stormed Holey Artisan Bakery Friday night.

"We have gunned down at least six terrorists and the main building is cleared but the operation is still going on," the commanding officer of Bangladeshi commandos, Lt. Col. Tuhin Mohammad Masud, told The AP three hours after the commandos launched the rescue operation.

Masud said the rescued include a Japanese, who was injured, and two Sri Lankans. Earlier, police said two hostages of Argentine and Bangladeshi descent have been rescued.

Masud said there are casualties, but did not provide details of the injuries.

Prime Minister Sheikh Hasina described the siege as a terrorist attack. ISIS has claimed responsibility for the attack.

"Anyone who believes in religion cannot do such an act," Hasina said. "They do not have any religion, their only religion is terrorism." The attack occurred during the Ramadan holy month.

Restaurant staffer Sumon Reza, who escaped, told The AP the attackers chanted "Allahu Akbar" (God is Great) as they entered the restaurant, initially opening fire with blanks.

Rezaul Karim, the father of a Bangladeshi businessman who was rescued along with his family, told the AP the attackers did not harm any hostage who could recite verses from the Islamic holy book, Quran.

Karim said his son told him that the attackers "did not hit people who could recite verses from the Quran. The others were tortured," he said.

Gunfire could be heard as Bangladesh forces moved in Saturday morning to end the standoff in the country's capital.

A man who witnessed the attack told ABC News he could hear the gunshots from his balcony.

Fayad Munain, who lives two streets away from the cafe where the attack occurred, said a cafe waiter told reporters that there were about six to eight gunmen who were all in their late 20s, Munain said. The waiter described the men as unshaven and said they fired three shots into the air before a "huge explosion."

Munain said he heard the explosion live and then about 5 seconds later on live television. He is an American with dual citizenship and is in Bangladesh visiting family, he said.

A Twitter post circulated by Amaq, an ISIS media outlet, said that ISIS “commandos” attacked “a restaurant frequented by foreigners in the city of Dhaka.”

U.S. State Department spokesman John Kirby could not confirm that ISIS was responsible for the attack.

The Associated Press, citing local media and a witness who escaped, said as many as nine gunmen stormed a restaurant. The gunmen took hostages and exchanged gunfire with security officials.

Kirby said in a statement, "Our embassy in Dhaka has confirmed 100 percent accountability of all official American personnel with no injuries reported. We are working with the local authorities to determine if any U.S. citizens and locally–employed staff were affected."

“We join with the people of Bangladesh in expressing our outrage at this brutal act of terrorism and offer our condolences to the friends and families of the victims, including Bangladeshi law enforcement officials who have been killed or injured responding to the attack," Kirby said. "We are in ongoing contact with the Government of Bangladesh as the situation continues to unfold. We have offered our assistance in their efforts to bring to justice those responsible for these attacks and to combat terrorism and violent extremism."

A White House official said President Barack Obama was briefed on the attack by his chief counterterrorism adviser Lisa Monaco. The president asked to be kept informed as the situation develops, said the official.