The United States says it is concerned about reports of loss of life after Algerian forces launched a military operation to free hostages seized by militants at a gas facility in eastern Algeria.
White House spokesman Jay Carney said he could not confirm the number of casualties or their identities.
"Right now our priority is determining the status of the Americans involved and gaining a full understanding of what took place. This is a fluid situation. We are seeking clarity from the Algerian government about this matter," Carney said.
A spokesman for British Prime Minister David Cameron said Britain was not given prior notice of the Algerian operation.
French President Francois Hollande said the Algerian hostage crisis shows the French intervention in neighboring Mali was justified.
Hollande also said the situation in Algeria seemed to have taken a "dramatic" turn and he was still seeking details.
Militants seized the In Amenas gas installation near the Libyan border on January 16, taking hostage tens of foreigners and Algerians.
The hostage-takers were demanding that France cease its operation against militants in Mali.
The Algerian communications minister said the operation to rescue the hostages was still going on late on January 17.
There were conflicting reports about the number of casualties.
Militants claimed 34 hostages and 14 kidnappers were killed.
Algerian official media reported four foreigners were rescued in the raid which they said also resulted in "several victims."
Previous reports said several dozen hostages including foreigners escaped earlier on January 17.
On January 16, the militants said they held 41 Westerners, including Americans, Japanese, and Europeans of various nationalities.
The Algerian interior minister has said the kidnappers were led by Mokhtar Belmokhtar, an Islamist militant who fought Soviet forces in Afghanistan in the 1980s and had set up his own group in the Sahara after falling out with other local Al-Qaeda leaders.
The In Amenas facility is run by giant oil firm BP jointly with Norway's Statoil and the Algerian state oil company.
Based on reporting by Reuters, BBC, and CNN