The death toll from the terrorist siege at a natural gas plant in the Sahara climbed past 80 on Sunday as Algerian forces searching the refinery for explosives found dozens more bodies, many so badly disfigured it was unclear whether they were hostages or militants, a security official said.

Algerian special forces stormed the plant on Saturday to end the four-day siege, moving in to thwart what government officials said was a plot by the Islamic extremists to blow up the complex and kill all their captives with mines sown throughout the site.

In a statement, the Masked Brigade, the group that claimed to have masterminded the takeover, warned of more such attacks against any country backing France's military intervention in neighbouring Mali, where the French are trying to stop an advance by Islamic extremists.

'The responsibility for these deaths lies squarely with the terrorists who launched a vicious and cowardly attack' —British Prime Minister David Cameron

"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.

Algeria said after Saturday's assault by government forces that at least 32 extremists and 23 hostages were killed. On Sunday, Algerian bomb squads sent in to blow up or defuse the explosives found 25 more bodies, said the security official, who spoke on condition of anonymity because of the sensitivity of the situation.

"These bodies are difficult to identify. They could be the bodies of foreign hostages or Algerians or terrorists," the official said.

In addition, a wounded Romanian who had been evacuated died, raising the overall death toll to at least 81.

"Now, of course, people will ask questions about the Algerian response to these events, but I would just say that the responsibility for these deaths lies squarely with the terrorists who launched a vicious and cowardly attack," British Prime Minister David Cameron said. Three Britons were killed and another three were feared dead.

Payback for supporting French attack on Mali, says rebel group

The dead hostages were also known to include at least one American as well as Filipino and French workers. Nearly two dozen foreigners by some estimates were unaccounted for.

It was unclear whether anyone was rescued in the final assault on the complex, which is run by the Algerian state oil company along with BP and Norway's Statoil.

Two private Algerian TV stations and an online news site said security forces scouring the plant found five militants hiding out and learned that three others had fled. That information could not be immediately confirmed by security officials.

Authorities said the bloody takeover was carried out Wednesday by 32 men from six countries, under the command from afar of the one-eyed Algerian bandit Moktar Belmoktar, founder of the Masked Brigade, based in Mali. The attacking force called itself "Those Who Sign in Blood."

The Masked Brigade said Sunday the attack was payback against Algeria for allowing over-flights of French aircraft headed to Mali and for closing its long border with Mali. In an earlier communication, the Brigade claimed to have carried out the attack in the name of al-Qaeda.

Armed with heavy machine guns, rocket launchers, missiles and grenades, the militants singled out foreign workers at the plant, killing some of them on the spot and attaching explosive belts to others.

Algeria's tough response questioned by some countries

Algeria's tough and uncompromising response to the crisis was typical of its take-no-prisoners approach in confronting terrorists, favoring military action over negotiation. Algerian military forces, backed by attack helicopters, launched two assaults on the plant, the first one on Thursday.

The militants had "decided to succeed in the operation as planned, to blow up the gas complex and kill all the hostages," Algerian Communications Minister Mohamed Said told state radio.

Norwegian Foreign Minister Espen Barth Eide said the terrorists had tried to blow up the plant on Saturday but managed only to start a small fire. "That's when they started to execute hostages, and the special forces intervened," Eide said. Norway's Statoil said five Norwegians were still missing.

Two British hostages, identified as Peter, left, and Alan, right, are seen after being released, in a street of Ain Amenas, near the gas plant where they have been kidnapped. (Anis Belghoul/Associated Press)

An audio recording of Algerian security forces speaking with the head of the kidnappers, Abdel Rahman al-Nigiri, on the second day of the drama indicated the hostage-takers were trying to organize a prisoner swap.

"You see our demands are so easy, so easy if you want to negotiate with us," al-Nigiri said in the recording broadcast by Algerian television. "We want the prisoners you have, the comrades who were arrested and imprisoned 15 years ago. We want 100 of them."

In another phone call, al-Nigiri said that half the militants had been killed by the Algerian army on Thursday and that he was ready to blow up the remaining hostages if security forces attacked again. An organization that monitors videos from radicals posted one showing al-Nigiri with what appeared to be an explosive belt around his waist.

The Algerians' use of forced raised an international outcry from some countries worried about their citizens.

But French Foreign Minister Laurent Fabius said Sunday on French television: "The terrorists ... they're the ones to blame."

David Plouffe, a senior adviser to U.S. President Barack Obama, said that al-Qaeda and al-Qaeda-affiliated groups remain a threat in North Africa and other parts of the world, and that the U.S. is determined to help other countries destroy those networks.

Speaking on "Fox News Sunday," Plouffe said the tragedy in Algeria shows once again "that all across the globe countries are threatened by terrorists who will use civilians to try and advance their twisted and sick agenda."