"We can now confirm the death of three U.S. citizens in the terrorist attack in Algeria: Victor Lynn Lovelady, Gordon Lee Rowan and Frederick Buttaccio," U.S. State Department spokeswoman Victoria Nuland said.
"We extend our deepest condolences to their families and friends. Out of respect for the families' privacy, we have no further comment. We are also aware of seven U.S. citizens who survived the attack. Due to privacy considerations, we have no further information to provide.
"As the president said, the blame for this tragedy rests with the terrorists who carried it out, and the United States condemns their actions in the strongest possible terms. We will continue to work closely with the government of Algeria to gain a fuller understanding of the terrorist attack of last week and how we can work together moving forward to combat such threats in the future."
Nuland did not provide hometowns for the victims.
Algerian Prime Minister Abdelmalek Sellal said in a televised news conference Monday the known hostage death toll was 37 victims from eight countries, with five people missing, all of them foreigners. Twenty-nine kidnappers also died.
Previously, officials had said the discovery of 25 charred bodies had brought the death toll to 81. The discrepancy wasn't explained and it wasn't known whether the bodies were those of hostages or captors.
Sellal said the Islamic militants who took the hostages last week came from countries including Egypt, Mali, Niger, Mauritania and Tunisia. Two of them were from Canada, the Canadian television network CTV reported. Canadian Foreign Affairs Minister John Baird said he was aware of reports of involvement by Canadian citizens.
Nearly 700 Algerians and 107 foreigners were freed or escaped from the gas field operated by British multinational oil and gas company BP PLC in partnership with Norway's Statoil ASA and Algerian government-owned energy company Sonatrach.
An Algerian warlord linked to al-Qaida claimed responsibility for the gas-plant siege, which started last Wednesday and ended Saturday with a raid by Algerian forces.
"We in al-Qaida announce this blessed operation," Mokhtar Belmokhtar is quoted by Mauritania's Sahara Media as saying in a video the West African news website did not immediately post online.
"We are ready to negotiate with Western countries and the Algerian regime, provided they stop their aggression and bombing of Mali's Muslims, especially in the Azawad territory," Sahara Media, monitored by United Press International, quoted Belmokhtar as saying.
The warfare in Mali involves several insurgent groups fighting for independence or greater autonomy for the northern Mali territory known as Azawad. The rebellious National Movement for the Liberation of Azawad declared Azawad independent of Mali last April.
France and some African countries are helping Mali try to stop the Islamic militants' advance.
Belmokhtar said the refinery attack was carried out by "40 mujahedin immigrants, most of them from Muslim countries and some even from Western countries."
He didn't identify the countries but called the purported jihadists "blood signatories."
A separate message attributed to the Masked Brigade warned of additional attacks against countries supporting French forces in Mali.
"We stress to our Muslim brothers the necessity to stay away from all the Western companies and complexes for their own safety, and especially the French ones," the statement said.
France has said its troops will stay in Mali "for as long as it takes to beat the terrorists." The French government said it expected to increase its forces to at least 2,500.