Officials say at least three Americans were being held hostage by the militants when the Algerian military mounted an rescue operation Thursday that reportedly resulted in casualties. The status of those three Americans is unclear.
Reports that as many as 35 hostages and 15 Islamist militants have been killed during a helicopter raid have not been confirmed. Estimates of the number of dead have varied wildly from four to dozens, but at least six have reportedly been killed.
"I think we have to prepare ourselves for the possibility of bad news ahead," said British Prime Minister David Cameron.
The Algerian government says there have been casualties, but won't confirm how many hostages have been killed, freed or are still being held. U.S. Secretary of State Hillary Clinton has been in contact with the Algerian prime minister.
"The security of our Americans who are held hostage is our highest priority, but of course, we care deeply about the other Algerian and foreign hostages as well," Clinton said.
The siege began Wednesday morning when an estimated 20 gunmen attacked a bus carrying a group of international workers. The bus was escorted by two cars of security teams, but during the strike, at least one worker was killed.
The terrorists moved next to the compound, jointly operated by BP and a Norwegian company, where they remain hunkered down with hostages from the U.S., Algeria, Norway, Japan, France and other countries.
Intelligence officials believe the attack was masterminded by Mokhtar Belmokhtar, a rogue al Qaeda leader who also runs an African organized crime network that reportedly has made tens of millions of dollars in ransom from kidnappings and smuggling. He is known as Mr. Marlboro because of his success smuggling diamonds, drugs and cigarettes.
U.S. officials say military assistance was offered to the Algerian government to help rescue the hostages, but it was refused. An unarmed American surveillance drone was overhead as the Algerian forces closed in, but officials could only watch and wait.
ABC News and the Associated Press contributed to this report.