The United States confirmed Monday that three of its citizens were among the foreign workers who died last week in an attack by Islamist hostage-takers on an Algerian gas plant.

State Department spokeswoman Victoria Nuland said seven more Americans survived the drama at the In Amenas site, and identified those killed as Victor Lynn Lovelady, Gordon Lee Rowan and Frederick Buttaccio.

Earlier, Algerian authorities had said 37 foreigners of eight different nationalities had been killed in the attack and subsequent four-day siege of the facility, which ended in a bloodbath on Saturday.

Nuland said she would not be giving more details about the US dead or the survivors, out of respect for the families' privacy, but cited President Barack Obama in blaming the Islamist militants for the bloodshed.

"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," she said.

"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."

The sprawling In Amenas gas plant was attacked on Wednesday by at least 32 heavily-armed fighters, who Algerian officials claim crossed into the country from neighboring Mali, where Islamist militants have launched a rebellion.

The gang took foreign workers hostage and Algerian forces responded with an uncompromising assault which only ended at the weekend.

Some foreign governments complained Algeria had kept them in the dark about an operation that many observers found hasty, but in public Washington and its allies have been cautiously supportive of Algeria's move.