Armed Somali group al-Shabab has claimed responsibility for the attack and ongoing siege of an upscale shopping mall in the Kenyan capital of Nairobi that has left at least 39 people dead and 150 injured.
Kenyan security forces have arrested one of the gunmen who attacked the mall on Saturday, the Kenyan presidency said on Twitter. Gunmen are still holding an unknown number of hostages inside the mall.
The east African country's head of police, David Kimaiyo, said via Twitter that several other assailants had been pinned down after soldiers and police moved into the mall to hunt down the attackers.
The French government confirmed that two of its citizens were killed in what President Francois Hollande described as a "cowardly attack." Two Canadians were also killed, including a diplomat, while the US government reported an undisclosed number of Americans injured.
The United Nations Security Council condemned the attack "in the strongest possible terms," and reminded Kenya that any response must comply with international human rights law.
On its own Twitter account, al-Shabab said there would be no negotiations with Kenyan officials over the standoff.
"10 hours have passed and the Mujahideen are still strong inside Westgate Mall and still holding their ground. All praise is due to Allah!", the group wrote.
Kenyan troops have been fighting al-Shabab in Somalia, where the anti-government group is seeking to impose Islamic law and has carried out numerous attacks.
Kenya has been hit by a spate of attacks including hand grenades and bombs since it sent troops to southern Somalia in late 2011 to attack al-Shabab bases.
"The border between Kenya and Somalia is very, very porous, and it's very difficult for authorities to control movement across the border," said Al Jazeera's Peter Greste, reporting from Nairobi. "There's also a very difficult relationship between Somali communities here and the security services; neither of them trust each other."
Call for unity
In a live television statement, Kenyan President Uhuru Kenyatta said that the country had "overcome terrorist attacks before."
"In fact, we have fought courageously and defeated them within and outside our borders - we will defeat them again," said Kenyatta.
"The way we lead our lives in freedom, openness, unity and consideration for each other represents our victory over those who wish us ill," said the president.
He reiterated the government's readiness to fight aggression domestically as well as externally, and urged Kenyans to "stand together and see this dark moment through" by donating blood, cooperating with authorities and comforting those affected by the day's violence.
Kenyatta added that he himself had lost close relatives in the mass shooting.
A gunbattle broke out inside the shopping centre as police and military forces struggled to evacuate shoppers and apprehend an unknown number of armed men.
A statement issued later by Kenya's Internal Security Minister Mutea Iringo said that the government was "fully in charge" of the situation at the mall.
A witness said the attackers opened fire and executed people after they threw grenades into the building at about midday local time (09:00 GMT).
Unverified witness accounts have indicated that the attackers targeted non-Muslims and foreigners.
The US State Department said Americans were reportedly among the injured in the attack, condemning the "senseless act of violence."
"We have reports of American citizens injured in the attack, and the US embassy is actively reaching out to provide assistance," spokeswoman Marie Harf said without giving further details.