The White House has condemned as "despicable" the attack that killed at least 39 people at a Nairobi shopping mall, vowing to assist Kenya's counterterrorism efforts.
Americans were among the injured - estimated at 150 - in the attack claimed by Somalia's al-Qaeda-linked al-Shabab militants, US Secretary of State John Kerry said.
Although no Americans are known to have died in the attack, he confirmed that one of those killed was the wife of a foreign national working for the US 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 added.
"Attacks like this can't change who we are, a people committed to peace and justice for all, but rather must reaffirm our determination to counter extremism and promote tolerance everywhere."
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The top US diplomat promised to help bring the perpetrators of the "abhorrent violence" to justice.
"The United States condemns in the strongest terms the despicable terrorist attack on innocent civilians today at the Westgate Shopping Mall in Nairobi, Kenya," US National Security Council spokeswoman Caitlin Hayden said in a statement.
"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."
Offering Washington's "deepest condolences" to victims' families and loved ones, Hayden vowed to "stand with the Kenyan people in their efforts to confront terrorism in all its forms, including the threat posed by al-Shabab".
Two Canadians, including a diplomat, are among the dead, Canada's Prime Minister Stephen Harper said.
"Canada condemns in the strongest possible terms this cowardly, hateful act that apparently targeted innocent civilians who were simply out shopping," Harper said in a statement.
He said Annemarie Desloges, a liaison officer with the Canada Border Services Agency at Canada's High Commission to Kenya, would be "remembered and honoured".
Paris said two French citizens were among those killed in the attack and Britain warned that some of its citizens were "undoubtedly" among the victims.
British Foreign Secretary William Hague said Britons had undoubtedly been caught up in the "callous and cowardly and brutal" assault by a heavily-armed gang who singled out non-Muslims.
Hague said "we should be ready for that and aware of that" and revealed the government's emergency response committee Cobra had met and a rapid deployment team was being sent to Kenya to help.
Hague said the UK's work to bring stability to Kenya and other countries neighbouring Somalia would continue.
"It will never be deterred or prevented by actions of this kind," Hague said.
"I know that will be the view of the Kenyan government and the people of Kenya as well."
He said al-Shabaab had brought "terror and great difficulty to Somalia".
"These are large numbers of entirely innocent people, that's why I say it is a particularly callous and cowardly and brutal attack," he said.
"Sadly significant numbers have been killed.
"So again I say that the thoughts of the UK are with the people of Kenya at this terrible moment."
The UN Security Council condemned "in the strongest possible terms" the attack.
"The members of the Security Council condemn in the strongest possible terms today's terrorist attack in Nairobi, which has caused numerous deaths and injuries," they said in a unanimous declaration also extending condolences to the victims and their families.
