WASHINGTON: The United States is condemning a deadly Taliban attack in Afghanistan, saying there is ''no possible justification'' for killing people who worked to help Afghans achieve a brighter future, whereas UN leader Ban Ki-moon vowed that the United Nations would maintain its work in Afghanistan despite the attack that killed four of its staff.

The US also is renewing its call for the Taliban to lay down its weapons and begin peace talks with the Afghan government.

White House spokesman Jay Carney says such a move is the surest way to bring a peaceful end to the conflict in Afghanistan.

The Taliban claimed responsibility for Friday's attack on a popular restaurant in Kabul, Afghanistan's capital and largest city.

Twenty-one people, including three American citizens working for the American University of Afghanistan, were killed.

The Taliban said the attack was in retaliation for an Afghan military operation earlier in the week.

The US ambassador to the United Nations, Samantha Power, slammed what she called a “craven attack.”

But she added in a statement that “the United States and the international community will not waver in its commitment to stand with the people of Afghanistan.”

The United States has promised to keep up its assistance to Afghanistan.

UNITED NATIONS UN Secretary-General Ban Ki-Moon called the attack ''totally unacceptable.'' and vowed Saturday that the United Nations would maintain its work in Afghanistan despite a Taliban suicide attack that killed four of its staff.

The four were among 13 foreigners and eight Afghans who died when Taliban suicide attackers set off a bomb at a popular Kabul restaurant.

Ban paid tribute to Basra Hassan, a US citizen of Somali origin, Nasrin Jamal of Pakistan, both health specialists for the UN children's agency, Khanjar Wabel Abdallah of Lebanon and Vadim Nazarov of Russia.

“This is another sad moment for the United Nations where our distinguished four colleagues have been killed by a terrorist attack in Kabul,” he said in New York.

While condemning what he called a “reckless” terror attack, Ban said the world body would not be deterred from its work in Afghanistan, where it is helping the government organize elections as international troops withdraw.

“As the United Nations mourns this terrorist attack and its victims, we remain committed to work for the peace, stability and development of Afghanistan,” the secretary general said.

“We fully support the transition of Afghanistan toward a better future in peace, development and security,” he added.

Deadliest attack on foreigners A Taliban attack against a popular Kabul restaurant killed 21 people, authorities said Saturday, making it the deadliest single attack against foreign civilians in the course of a nearly 13-year U.S.-led war there now approaching its end.

The attack comes as security has been deteriorating and apprehension has been growing among Afghans over their country's future as U.S.-led foreign forces prepare for a final withdrawal at the end of the year.

Afghan President Hamid Karzai, who is deferring signing an agreement allowing US forces to stay past the planned withdrawal until after the country's April 5 presidential election, criticized America while condemning the attack.

''If NATO forces and in the lead the United States of America want to cooperate and be united with Afghan people, they must target terrorism,'' he said without fully elaborating on what America should be doing. He added that the US had followed a policy that ''was not successful in the past decade.''

The dead from Friday's assault against La Taverna du Liban included 13 foreigners and eight Afghans, all civilians.

The US Embassy in Kabul said late Saturday that three Americans were killed.

Previously, those identified included two US citizens working for the American University of Afghanistan and a victim identified by the United Nations as Basra Hassan, a Somali-American working as a nutrition specialist for Unicef.

Others identified were two Britons, development specialist Dharmender Singh Phangura and close protection officer Simon Chase, two Canadians who worked for a financial services firm, two Lebanese, a Danish police officer, a Russian, and a Malaysian. Phangura, who along with the Malaysian worked as an adviser for Adam Smith International, was to run as a Labour Party candidate in upcoming elections for the European Parliament.

Also among the dead were the International Monetary Fund's representative, Khanjar Wabel Abdallah of Lebanon; Nasreen Khan, a Unicef health specialist from Pakistan, and Vadim Nazarov, a Russian who was the chief political affairs officer at the UN Mission in Afghanistan.

Nazarov was one of the UN's most experienced officials, fluent in the country's languages and with experience dating back to the 1980s.

The attack was condemned by the UN Security Council, Nato, the White House and the European Union.

''There is no possible justification for this attack, which has killed innocent civilians, including Americans, working every day to help the Afghan people achieve a better future with higher education and economic assistance,'' the White House said in a statement Saturday.

Insurgents have frequently targeted foreign interests around the country and in Kabul.

The deadliest previous attack against foreign civilians was in Sept 8, 2012, when nine civilian employees of a private aviation company were killed in a suicide attack near the Kabul airport.

They included eight South Africans and a Kyrgyz.

The assault began Friday with a suicide bomber detonating his explosives at the front door of the restaurant, located in an area housing several embassies, non-governmental organisations and the homes and offices of Afghan officials.

As chaos ensued, the two other attackers entered through the kitchen and began shooting. They were later killed by security guards

The Taliban claimed responsibility for the attack, saying it was in reprisal for an Afghan military operation earlier in the week against insurgents in eastern Parwan province, which the insurgents claimed killed many civilians.

Afghan officials previously said that attack killed a Taliban leader, three of his family members, seven Taliban fighters and five civilians in a neighboring home from which insurgents were also firing on the Afghan commandos.

The Taliban frequently provide exaggerated casualty figures.

''The target of the attack was a restaurant frequented by high-ranking foreigners,'' Taliban spokesman Zabihullah Mujahid said in an emailed statement. He said the attack targeted a place ''where the invaders used to dine with booze and liquor in the plenty.''

He described the ''revenge attack'' as having delivered a ''heavy admonitory blow to the enemy which they shall never forget.''

The restaurant, like most places frequented by foreign diplomats, aid workers, journalists and businessmen in the war-weary country, has no signs indicating its location and is heavily secured.

It sits on a small side street just off a bumpy semi-paved road in a house with low ceilings and an enclosed patio but has no windows.

Bags of dirt are piled up around it to act as blast walls and guests must go through a series of steel airlocks, where they are searched, before entering the premises