Kabul, Afghanistan (CNN) -- The Obama administration condemned Saturday the Taliban's attack on a Kabul restaurant that killed 21 people, mostly foreigners, as revenge for an airstrike that caused civilian Afghan deaths.
Among the dead are Americans, Canadians, Britons and Afghans.
Two of the three Americans killed were identified Saturday. They are Lexie Kamerman and Alexandros Petersen, both employees at the American University of Afghanistan, the university said.
A third American has not yet been named.
"She was an amazing young woman -- smart, strong, beautiful, funny, stubborn and kind. And fearless," Kamerman's family said in a statement provided by her aunt, Julie Pfeffer.
"She took the job at the American University of Afghanistan to help the young women of Afghanistan get an education and take their rightful place as leaders in Afghan society," the family said. "As you could probably guess, her death is a shock to us all and we can't imagine a moment going forward when she won't be desperately missed."
Laura Coffman, a family friend who had been Kamerman's water polo coach at the Latin School of Chicago, said Kamerman had recently visited her mother in Chicago.
She said Kamerman was known as someone who never shied away from a challenge or visiting a place considered dangerous.
"I don't think that would make her hesitate a second," Coffman told CNN. "What she would have said was: Just because it's dangerous doesn't mean she shouldn't be there."
U.S. officials excoriated the attack
The bomb and gunfire attack Friday occurred at a Kabul restaurant frequented by foreign workers from nongovernmental organizations.
"There is no possible justification for this attack, which has killed innocent civilians, including Americans," White House press secretary Jay Carney said in a prepared statement. "We call again on the Taliban to put down their arms and begin peace talks, which is the surest way to end the conflict in a peaceful manner.
A U.S. State Department official said that three Americans died in the attack.
Four of the people killed were affiliated with the United Nations: three U.N. staff members and a Lebanese national with the International Monetary Fund, said Ari Gaitanis, a U.N. spokesman.
The attack was "a heinous and cowardly act targeting innocent people working for a brighter future for Afghanistan," National Security Adviser Susan Rice said on Twitter.
The Taliban claimed responsibility for the assault as payback for an airstrike in Parwan province that caused civilian casualties this week.
Suicide bombing and gunfire
A suicide attacker detonated his explosives at the gate of the restaurant in the evening. Two armed men rushed in and opened fire at patrons, many of them from overseas, said Deputy Interior Minister Mohammed Ayoub Salangi.
There were 13 foreigners in total among the dead, including four women, the Afghan Interior Ministry said. The eight Afghans killed in the attack included one woman. Two other people were wounded.
One of the two killed American employees of American University of Afghanistan had recently joined the political science faculty, the school told CNN. The other was a member of the student affairs staff.
"We are devastated by the news," university President Michael Smith said in a prepared statement. "Our deepest sympathies go out to the families and to the AUAF community."
The university said it was planning a memorial service and moment of silence for the employees killed in the assault on La Taverna du Liban, a popular Lebanese eatery known among expatriates to be one of the more secure establishments in the city.
"Such senseless violence flies in the face of the sentiments of our students and the Afghan people who share our grief," Smith said. "We will pause to honor the courageous service of our colleagues as we continue to provide the high quality university education for which our students are so grateful."
2 Britons, 2 Canadians among the dead
Two British victims of the assault have now been named.
One is Del Singh, a candidate for the upcoming elections for the European Parliament from the southeast of England, a spokesman for the opposition Labour Party said, adding that a full tribute was being arranged.
"Del spent over 10 years carrying out vital work on development projects in Afghanistan, Kosovo, Sudan, Sierra Leone and other countries," Labour Party leader Ed Miliband said. "He dedicated his life to working with people across the world who needed his support," he said.
The Foreign Office named the second victim as Simon Chase, but provided no further information Saturday morning.
The Russian Foreign Ministry said Saturday that one of the U.N. workers was a Russian national.
A Canadian foreign affairs department spokeswoman said two of her country's nationals -- Martin Glazer and Peter McSheffrey -- died in the attack.
Both worked for a consulting and auditing firm in nongovernmental organizations' projects in Afghanistan, said spokeswoman Caitlin Workman. They did not work directly for Canada's Department of Foreign Affairs, Trade and Development.
Taliban claim attack as revenge
Afghan security forces killed the two gunmen in a shootout.
The restaurant is near the offices of many nongovernmental organizations, said Hashmat Stanikzai, a Kabul police chief spokesman.
The Afghan interior minister condemned the attack in an online statement, saying "these heinous acts go against the values of Islam and the values of peaceful Afghans. These attacks also demonstrate an extreme level of atrocity by terrorists on innocent and defenseless civilians. "
U.N. Secretary-General Ban Ki-moon condemned the violence, saying through a spokesman such attacks "are completely unacceptable and are in flagrant breach of international humanitarian law."
Among the killed was Vadim Nazarov, a senior political officer with the U.N. Assistance Mission in Afghanistan, a U.N. website said.
UNICEF identified two employees as part of the death toll: Basra Hassan, a nutrition specialist, and Nasreen Khan, a health specialist.
Christine Lagarde, the IMF's managing director, posted a statement online mourning the death of her agency's representative in Afghanistan, Wabel Abdallah. The 60-year-old Lebanese national was named to that position in June 2008.
"We at the fund are all devastated," Lagarde said.
Afghanistan continues to be the site of sporadic violence, much of it blamed on militants tied to the Taliban. The terror group ruled the country before the U.S.-led invasion after the September 11, 2001, terrorist attacks.
The international community has been extensively engaged in Afghanistan for more than a decade both with military troops and NGOs.
Friday night's attack was a "huge shock" to those working there, said Paul Ross, the IMF's mission chief for Afghanistan, but it won't deter them from continuing their work.
"I think that many of the people, like Wabel, are dedicated to trying to help countries develop and prosper," Ross said. "That's really part of their life mission statement. And that's what makes them go to places that are difficult to visit."
CNN's Masoud Popalzai reported from Kabul, and CNN's Greg Botelho wrote it from Atlanta. CNN's Ben Brumfield, Jessica King, Lindsay Isaac and Neda Farshbaf contributed to this report.