The West faces a decades-long battle to defeat al-Qa'ida in North Africa, David Cameron warned today, as he signalled a dramatic shift in the UK's fight against terrorism.

The heads of MI5, MI6, GCHQ and the Chief of the Defence Staff will gather on Tuesday to begin planning Britain’s response to the burgeoning terror threat from Saharan Africa.

Britain will offer money, military co-operation and security training to African states to head off the advance of Islamist radicalism.

Special forces are understood to be preparing to hunt down the jihadist leader behind the siege and hostage killings in Algeria, Mokhtar Belmokhtar.

Britain will use its chairmanship of the G8 to focus militarily and diplomatically on the Sahara region, following the hostage crisis which claimed the lives of up to six Britons. One Middle East expert likened the long-term impact of the atrocity in Algeria to the 9/11 attacks.

Following the end of the four-day stand-off at the BP gas plant at In Amenas, Algerian forces discovered 25 more bodies and took five militants alive. The death toll had previously been put at 23 hostages and 32 captors.

Three Britons have been confirmed among the dead and another three are feared to have been killed during the siege, which ended with a shoot-out on Saturday. Tonight 46-year-old Paul Thomas Morgan was the first British victim to be named by the Foreign and Commonwealth Office.

Kenneth Whiteside, an engineer from Glenrothes in Fife, and Garry Barlow, a BP systems supervisor from Merseyside, are also understood to be among the dead. Another UK resident was also believed to have been killed.

Twenty-two other British nationals have arrived home, many with chilling stories of how they evaded capture by jihadists belonging to an al-Qa'ida splinter group styling themselves Those Who Sign In Blood.

Alan Wright, from Aberdeenshire, told of how he hid in an office for 24 hours before joining Algerian workers who cut their way through a perimeter fence and fled.

Mr Cameron will update MPs on the attack today and hold a meeting of Whitehall's emergency Cobra committee to consider the implications of the attack.

French forces – with support from Britain – are attempting to oust insurgents from northern Mali, amid fears that neighbouring countries including Niger and Mauritania could fall under their influence.

As the French Defence Minister, Jean-Yves Le Drian, described the hostage-taking as an "act of war", Belmokhtar was reported to be "ready to negotiate" in return for an end to the action in Mali.

Last night Mauritanian news website Sahara Media said Belmokhtar had claimed responsibility in the name of al Qa'ida for the hostage-taking in a video. He had said: "We in al Qa'ida announce this blessed operation. We are ready to negotiate with the West and the Algerian government provided they stop their bombing of Mali's Muslims. We had around 40 jihadists, most of them from Muslim countries and some even from the West."

A BP spokesman would not comment on reports in Algeria that Belmokhtar's men had infiltrated the gas plant as drivers, cooks and guards working on short-term contracts.

Mr Cameron spelt out the scale of the challenge posed by al-Qa'ida-affiliated groups operating in the region. "It will require a response that is about years, even decades, rather than months," he said. "And it requires a response that is painstaking, that is tough but also intelligent, but above all has an absolutely iron resolve. And that is what we will deliver over these coming years.

"What we face is an extremist, Islamist, al-Qa'ida-linked terrorist group. Just as we had to deal with that in Pakistan and in Afghanistan, so the world needs to come together to deal with this threat in North Africa... We need to work with others to defeat the terrorists and to close down the ungoverned spaces where they thrive with all the means that we have."

The Government has not ruled out giving extra help to the French-led operation in Mali.

However, Whitehall sources said the terrorist threat in the region would ultimately be best tackled by diplomatic means. Britain is to beef up its presence in nations where the UK historically had a limited presence and to liaise more closely with Paris over the challenges faced by the traditionally Francophone area.

Abdelasiem el-Difraoui, an al-Qa'ida expert with the Berlin Institute for Media and Communications Studies, told a French newspaper that the hostage-taking would for France make as "a huge bang as strong as September 11".

The French Government distanced itself from suggestions among other nations caught up in the hostage crisis that Algeria's response was "heavy-handed".

President François Hollande said: "When so many hostages have been taken and when the terrorists are ready to murder them in cold blood, I think the Algerian approach was the best one."

Britons in the desert

Garry Barlow: Semtex was strapped to his chest

Garry Barlow, 49, was a systems supervisor for BP Exploration Algeria, Statoil and Sonatrach JV. He lived in the Mossley Hill area of Liverpool with his wife Lorraine, and sons Scott, 17, and Paul, 15.

He had been working in In Amenas since October 2011, and had worked previously for Addax Petroleum and Shell EP on the west coast of Central Africa.

He was captured with some of his colleagues including 29-year-old project services contracts administrator Mark Grant, who is believed to have survived the ordeal.

Initial reports suggested Mr Barlow was safe and well and was being repatriated by the Foreign Office, but he is now thought to have died as Algerian troops tried to regain control of the compound.

The last his wife heard from him was a message in which he said: "I'm sitting here at my desk with Semtex strapped to my chest. The local army have already tried and failed to storm the plant and they've said that if that happens again they are going to kill us all."

Paul Morgan: Former soldier died fighting

The first British victim of the Algerian hostage crisis was described last night as a "true gentleman" who "loved life and lived it to the full".

Paul Morgan, 46, from Liverpool, a former soldier with the French Foreign Legion, reportedly "went down fighting" when the bus he was travelling in was attacked by the kidnappers last Wednesday.

His mother Marianne and partner Emma Steele, 36, paid tribute to him: "Paul died doing the job he loved. We are so proud of him and so proud of what he achieved in his life. He will be truly missed."

Kenneth Whiteside: Shot as army stormed compound

Kenneth Whiteside had been living in Johannesburg with his wife and two daughters but was originally from Glenrothes in Fife.

An Algerian colleague at the plant is said to have witnessed the BP project services manager "being shot" by his captors as commandos stormed the compound.

The 59-year-old was educated at Auchmuty High School and studied engineering at Glenrothes Technical College between 1970 and 1974.

Friends posted tribute messages on his Facebook account on Saturday. Steward Goodwin in South Africa wrote: "How will we understand this? My heartfelt condolences go to the family and friends who are trying to come to terms with this senseless murder."

Billy Hunter wrote: "We'll always remember him and his bagpipes." "It's hard to understand such senseless waste of life," added Joe McMahon.

Mr Whiteside had been working at the Algerian plant since September 2005.

Alan Wright: Perilous escape from terrorists

Alan Wright yesterday described his daring break for freedom from the In Amenas compound.

Mr Wright, who has two children and is from Portsoy, Aberdeenshire, hid in an office before joining Algerian colleagues who cut their way through a fence and escaped.

He said he was given a hat to make him "look less expat" and the group made a break for it. "The wire makes such a noise when it breaks and you knew it travelled to where the terrorists were," said Mr Wright, 37, a BP employee. He said that after walking about a kilometre into the desert he could see a military checkpoint with personnel pointing guns in their direction.