© Photograph: ReutersPaul Thomas Morgan, who was killed during the hostage crisis at the In Amenas gas plant and the first British victim to be identified.
At least nine Japanese nationals reported killed in attack claimed by veteran Islamist fighter on behalf of al-Qaida.

The death toll from the four-day siege at an Algerian gas plant deep in the Sahara has risen to at least 81, with nine Japanese nationals also reported killed in an attack claimed by a veteran Islamist fighter on behalf of al-Qaida.

The Algerian prime minister, Abdelmalek Sellal, is expected to give details at a news conference on Monday about one of the worst international hostage crises in decades, which left British, American, French, Japanese, Norwegian and Romanian workers dead or missing.

A security source said on Sunday that Algerian troops had found the bodies of 25 hostages, raising the total number of hostages killed to 48 and the total number of deaths to at least 80. He said six militants were captured alive and troops were searching for others.

That number climbed further on Monday when a Japanese government source said the Algerian government had informed Tokyo that nine of its citizens had been killed, the biggest toll so far among foreigners at the plant.

Veteran Islamist fighter Mokhtar Belmokhtar claimed responsibility for the attack on behalf of al-Qaida.

"We in al-Qaida announce this blessed operation," he said in a video, according to Sahara Media, a regional website. He said about 40 attackers participated in the raid, roughly matching the government's figures for fighters killed and captured.

The fighters came out of the desert and seized the facility on Wednesday, capturing a plant that produces 10% of Algeria's natural gas exports and residential barracks nearby.

They demanded an end to French air strikes against Islamist fighters in neighbouring Mali that had begun five days earlier. However, US and European officials doubt such a complex raid could have been organised quickly enough to have been conceived as a direct response to the French military intervention.

David Cameron has warned that the fight against terrorism in north Africa could last for decades as he confirmed that six Britons and a British resident were believed to have died.

The prime minister said the seizure of the In Amenas gas facility by a group of international jihadists was a stark reminder of the threat from terrorism the world over. He pledged a global response to what he described as a global threat.

"It will require a response that is about years, even decades, rather than months," Cameron said. "Tragically, we now know that three British nationals have been killed, and a further three are believed to be dead. And a further British resident is also dead."

He said the "whole country" would join him in sending condolences to the victims' families.

Cameron was speaking hours before the first of the British victims was identified on Sunday night as Paul Thomas Morgan, 46. In a statement issued by the Foreign Office, his family described him as a "true gentleman" who "loved life and lived it to the full" and "died doing the job he loved".

Morgan was reported to have been a veteran of the 1991 Gulf war and former French Foreign Legionnaire working as a liaison between workers and local security staff. Originally from Liverpool but living in Spain, he was said to have been in a bus going to the airport to return home following a one-month stint when the attack took place.

The prime minister's statement is a recognition that the Arab spring has irrevocably changed the dynamics of the wider region, ushering in new democratic possibilities but also dangers from unpredictable, ruthless and well-organised extremist groups.

Asked if the current al-Qaida threat to north Africa was comparable to the threat in Afghanistan a decade ago, Cameron replied: "It is different in scale but there are similarities."

He added: "What we face is an extremist Islamist violent al-Qaida-linked terrorist group - just as we have to deal with that in Pakistan and Afghanistan."

A further 22 Britons involved in the crisis at the In Amenas gas facility have returned to the UK, the Foreign Office said. The Algerian authorities said they had captured alive six members of the militant group during a search of the complex.

Amid confusion about what precisely happened on Saturday, Algerian forces searching the refinery for explosives on Sunday found dozens more bodies, many so badly disfigured it was unclear whether they were hostages or militants, a security official said.

Speaking on Sunday, Alan Wright, who survived the attack, described how he hid in an office after the jihadists stormed the compound last Wednesday.

Wright, 37, a BP employee, said he eventually joined Algerian colleagues who fled by cutting their way out through a fence.

"If you have been captured, there's pretty much no escape and it is going to take a miracle to get you out," he told Sky News. He said the expatriates were surprisingly calm as word of the attack spread with everyone "going into safe mode".

The most chilling moment came when one of the hostage-takers walked past their building and said good morning in a friendly Arabic voice, trying to trick the foreigners out, he said. "That was the first moment when we thought we are in big trouble here," Wright said.

Speaking on the BBC's Andrew Marr Show, William Hague, the foreign secretary, said it was "quite likely" some of the Britons were executed by the hostage-takers, who identified themselves as the Signers in Blood - a splinter group of al-Qaida in the Islamic Mahgreb (AQIM).

Communications minister Mohamed Said admitted the final death toll "strongly risked being revised to a higher figure". Earlier, the Algerian authorities had said 107 foreign hostages and 685 Algerian nationals were freed.

Among those reportedly shot dead was Abdul Rahman al-Nigeri, a terrorist from Niger, who is thought to have led the jihadist group that attacked the desert plant, initially ambushing a convoy of buses leaving with expats for the airport. The attackers came from six different countries, Said said. At least eight Algerian hostages died, together with nine Japanese nationals and one from France. One American was confirmed dead.

Statoil, the Norwegian company that runs the plant alongside Britain's BP and Algeria's state oil company, said five Norwegian staff were missing.

France's foreign minister, Laurent Fabius, also expressed solidarity with Algeria, which has battled a major Islamist insurgency since the 1990s. He said: "Faced with terrorism, we have to be unwavering. These are killers, they pillage, they rape, they sack."

The situation was dreadful, he said, adding: "The Algerians know at what point terrorism is an absolute evil."

Fabius rejected suggestions that he was being soft on Algeria because France needed - and had been given - permission by Algiers to overfly its territory during France's military intervention in neighbouring Mali.

President Barack Obama said the US would ask Algeria for a full explanation of what happened, but said: "The blame for this tragedy rests with the terrorists."

Cameron will chair a meeting on Monday of the government's crisis response committee, Cobra, to discuss Algeria, and will make a statement on the attack in the Commons. The National Security Council is meeting on Tuesday to discuss the threat posed by AQIM. "We have already been looking a lot at the threat posed by AQIM, but more work will need to be done," said a source.