Islamists who held several hostages at an Algerian gas plant sparking a bloody commando raid threatened Friday to stage more attacks, according to a spokesman cited by the Mauritanian news agency.

"Taking into account the suffering of the Algerian people, we promise the regime in place that there will be more operations," a spokesman for the "Signatories in Blood" told ANI.

The Algerian army is still pursuing "terrorists" at the Algerian gas plant and still searching for hostages at the site, British Prime Minister David Cameron told parliament on Friday.

Cameron said the first stage of the military operation was complete, "but this is a large and complex site and they are still pursuing terrorists and possibly some of the hostages in other areas of the site".

Price pressures on the global oil market have suddenly tightened, and the deadly hostage drama at a gas field in Algeria puts a "dark cloud" over the country's entire energy sector, the International Energy Agency warned on Friday.

The IEA said that a dominant factor in the global energy market now "has a lot to do with political risk writ large, and not just in Syria, Iran, Iraq, Libya or Venezuela", and including regulatory risks.

Algeria came under mounting international criticism on Friday as fears grew for dozens of foreign hostages still unaccounted for after a deadly commando raid against their Islamist captors at a desert gas field.

Algerian Communications Minister Mohamed Said said Thursday's air and ground assault freed a "large number" of hostages, but Algerian news reports said nearly 600 of those rescued were Algerian workers but just a handful were among the 41 foreigners reported seized.

Washington was Thursday trying to determine the fate of U.S. hostages caught in the crossfire of an Algerian military rescue bid, and urged its embassies and U.S. businesses to tighten security.

"We are certainly concerned about reports of loss of life. And are seeking clarity from the government of Algeria," White House spokesman Jay Carney said.

Mokhtar Belmokhtar, the one-eyed Islamist whose men seized dozens of hostages in a deadly attack on an Algerian gas field, is a wily desert fox branded as a terrorist by some but perhaps just a common brigand.

He was born in 1972 in the ancient desert city of Ghardaia, 600 kilometers (370 miles) south of the Algerian capital, noted for its date production and manufacture of rugs and fabrics.

An Algerian air strike killed 34 hostages, some of them Westerners, and 15 of their Islamist kidnappers at a desert gas field on Thursday, the ANI news agency quoted a spokesman for the kidnappers as saying.

"Thirty-four hostages and 15 kidnappers were killed in an (air) raid by the Algerian army," the spokesman said in remarks that have not been independently confirmed.

France cannot confirm that its citizens are among foreign hostages taken by Islamic extremists at an Algerian gas field, government spokeswoman Najat Vallaud-Belkacem said Thursday.

"We are not able, at this current time, to confirm the presence of French citizens among the hostages," she told Radio Classique.

The United States on Wednesday confirmed that American citizens are among hostages being held in Algeria after an attack by Islamist militants near a southern gas field.

"The best information that we have at this time is that U.S. citizens are among the hostages," State Department spokeswoman Victoria Nuland said, but she added that for the safety of the people concerned she would not give further details on how many were held and who they were.

Algeria, which has opened its airspace to French warplanes hitting Islamists in northern Mali, was targeted Wednesday in a deadly revenge attack in which militants seized 41 Western hostages.

The Islamists, who said they entered Algeria from northern Mali, told Mauritanian media they were holding 41 Westerners, among them French, British and Japanese citizens, as well as seven Americans, at a southern gas field.