Gunmen who killed dozens of people at a crowded shopping mall frequented by Westerners in Kenya's capital were holding hostages early Sunday, police said.
At least 39 people were killed in the initial attack in Nairobi on Saturday, Kenyan President Uhuru Kenyatta said. The U.S. State Department said American citizens were reportedly among the more than 150 injured.
The militant Islamic group al-Shabab, based in neighboring Somalia, claimed responsibility for the attack via Twitter, saying it was in retaliation "for the lives of innocent Muslims" killed by Kenyan forces leading an African Union offensive against al-Shabab.
Manish Turohit, 18, who hid in a parking garage for two hours before leaving in a line of 15 people who exited with their hands in the air to avoid being shot, told al-Jazeera that gunmen carrying AK-47s and wearing vests with hand grenades on them stormed into the mall.
"They just came in and threw a grenade," he said. "They were shouting and firing."
A North Carolina woman told NBC affiliate WAVY of Portsmouth, Va., by Skype that she was at the Westgate mall for lunch when the attack began.
"We stood up and started to turn, and we heard machine guns," said Bendita Malakia, 30, of Elizabeth City, who had moved to Nairobi in July to work at a financial company. "Then, we started to run and there was a second explosion, which knocked us on the ground."
Malakia said she and a friend took shelter in a store with dozens of other people, and the store manager pulled down gates to block the attackers.
"While we were back there you could hear them methodically going from store to store, talking to people, and asking questions," Malakia said. "They were shooting, screaming. Then it would stop for a while and they would go to another store."
Another unnamed witness quoted by The Associated Press said the gunmen told Muslims to stand up and leave and that non-Muslims would be targeted.
The New York Times reported that a confidential United Nations security report said two squads of gunmen attacked on different floors of the mall.
After the initial onslaught, police helicopters circled above the mall as armed police shouted "get out! get out!" and scores of shoppers fled the building, Reuters reported.
Joseph Ole Lenku, Kenya's secretary for defense and internal affairs, said government security forces had taken over the mall and that the situation was "under control," but gunfire was heard hours later and al-Shabab claimed its fighters were still inside battling government forces.
Early Sunday, the standoff was focused on a supermarket at the Westgate Mall, a new Israeli-owned shopping center, the AP reported. Kenyatta called the situation "delicate" and said a top priority was to safeguard hostages.
Kevin Jamal told Reuters that his sister was taken hostage. "I want her to come out alive," he said.
In a televised address to the nation late Saturday, Kenyatta said at least 39 people had been killed, including some of his "very close family members," and 150 injured.
"The despicable perpetrators of this cowardly act hoped to intimidate, divide and cause despondency amongst Kenyans,'' he said. "We have overcome terrorist attacks before. We will defeat them again."
In Washington, the White House and State Department condemned the attack and promised to help the Kenyan government bring the perpetrators to justice.
"We will continue to stand with the Kenyan people in their efforts to confront terrorism in all its forms, including the threat posed by al-Shabab," the White House said in a statement. "This cowardly act against innocent civilians will not shake our resolve."
In a statement issued Saturday evening, a State Department official said the agency has received "reports that four U.S. citizens have been injured in the attack. ... Our consular officers have been in contact with the victims and have been providing all appropriate consular assistance." The official declined further comment, citing privacy concerns.
Federal sources told NBC News that the FBI had accounted for all U.S. Embassy personnel, none of whom were believed to have been at the mall at the time.
In a later update, however, the State Department said one of the dead was the wife of a foreign service national working for the U.S. Agency for International Development. U.S.AID is part of the State Department and works out of the U.S. Embassy.
Canadian Prime Minister Stephen Harper said two Canadian citizens were among the dead, including a diplomat. He did not identify either. France also said two of its citizens were dead.
Al-Shabab tweeted that the attack was “retribution.” The group vowed in late 2011 to carry out a large-scale attack in Nairobi in retaliation for Kenya's sending troops into Somalia to fight the Islamic insurgents.
Formally named Harakat al-Shabab al-Mujahideen (HSM), but commonly known as al-Shabab, the group has been fighting to overthrow the Somali Transitional Federal Government (TFG), which is supported by the African Union and Western nations, since its founding in 2004. Designated as a terrorist organization by the U.S. and other Western governments, the group is aligned with al Qaeda and aims to establish an Islamist government in Somalia.
NBC News' Richard Esposito, Charlene Gubash, Andrea Mitchell and Kristen Welker; WNBC's Jonathan Dienst; and Reuters and The Associated Press contributed to this report.