Early Sunday morning in Kenya -- 12 hours after a deadly shooting attack left at least 39 people dead in an upscale mall in the capital Nairobi -- gunmen from the armed Somali group al-Shabab remained holed up inside with an unknown number of hostages, according to The Associated Press.
Kenyan President Uhuru Kenyatta, who said members of his own family were among the dead, called the security operation under way "delicate" and said a top priority was to safeguard hostages.
Al-Shabab claimed responsibility for the Saturday shooting, which also injured more than 150 people.
The Somali group confirmed to Al Jazeera that it was behind the deadly attack, which began shortly after noon local time Saturday, after posting a series of ominous statements on Twitter.
Throughout Saturday, security forces searched for shoppers hiding within the Westgate mall, popular with wealthy Kenyans and expatriates.
According to Secretary of State John Kerry, American citizens were among those injured. He said there were no reports of American deaths, but one of the people killed was the wife of a foreign service national who is working for the U.S. Agency for International Development.
"Today's terrorist massacre of so many innocents is a heartbreaking reminder that there exists unspeakable evil in our world which can destroy life in a senseless instant," Kerry said in a release. "We have offered our assistance to the Government of Kenya and stand ready to help in any way we can."
The State Department also said that according to local media reports about 10 gunmen attacked the mall.
U.S. National Security Council spokeswoman Caitlin Hayden issued a statement condemning the attack.
"The perpetrators of this heinous act must be brought to justice, and we have offered our full support to the Kenyan government to do so," Hayden said.
Kenyan military and police surrounded the mall Saturday, which had been hosting a children's day event, and helicopters flew overhead as gunmen remained inside hours after the attack occurred.
President Kenyatta said via Twitter Saturday that one of the gunmen had been arrested. Later Saturday, he reported that the gunman had died from bullet wounds, according to Reuters.
Bursts of gunfire were heard outside the shopping mall, where shoppers ran for cover and left cars abandoned. Witnesses said six grenades also went off along with lobbies of AK-47 gunfire.
The gunmen announced that non-Muslims would be targeted, said Elijah Kamau, who was at the mall at the time of the attack.
"The gunmen told Muslims to stand up and leave. They were safe, and non-Muslims would be targeted," Kamau told AP.
Jay Patel, who sought cover on an upper floor in the mall when the shooting began, said that when he looked out of a window onto the upper parking deck of the mall he saw the gunmen with a group of people.
Patel said that as the attackers were talking, some of the people stood up and left, while others were shot.
The gunmen carried AK-47s and wore vests with hand grenades on them, said Manish Turohit, 18, who hid in a parking garage for two hours.
"They just came in and threw a grenade. We were running and they opened fire. They were shouting and firing," he told AP after marching out of the mall in a line of 15 people who all held their hands in the air, in an apparent attempt to not be shot.
Rob Vandijk, who works at the Dutch embassy, said he was eating at a restaurant inside the mall when attackers lobbed hand grenades inside the building. He said gunfire then burst out and people screamed as they dropped to the ground.
It appears the attack began at the outdoor seating area of Artcaffe at the front of the mall, witnesses said.
Kenya -- which sent troops into Somalia in late 2011 to pursue al-Qaeda-linked fighters -- has suffered a string of retaliatory gun and grenade attacks claimed by the armed group al-Shabaab.
Last month, four Kenyan police officers were shot dead in the border county of Garissa when 40 heavily armed men, suspected of belonging to al-Shabab, attacked a police post, a senior regional government official said.
In July, al-Shabab released two Kenyan government officials it had seized in a 2012 cross-border attack, after holding them hostage in Somalia for more than a year.
Al Jazeera and wire services