6 Britons, UK resident believed killed in Algeria

"Tragically, we now know that three British nationals have been killed, and a further three are believed to be dead. And also a further British resident is also believed to be dead," he said in a statement.

"I know the whole country will want to join me in sending our sympathies and condolences to the families who have undergone an absolutely dreadful ordeal, and now face life without these very precious loved ones."

The Foreign Office later named one of the British men killed as Paul Morgan.

"Paul was a true gentleman, a family man, he very much loved his partner Emma, his mum, brothers and sister, of whom he was very proud," said the statement, issued on behalf of his 65-year-old mother, Marianne, and his 36-year-old partner, Emma Steele.

"He loved life and lived it to the full. He was a professional man proud to do the job he did and died doing the job he loved.

"We are so proud of him and so proud of what he achieved in his life. We are devastated by Paul's death and he will be truly missed," it added.

Cameron stressed that responsibility "lies squarely with the terrorists who launched this vicious and cowardly attack", and said he would put the issue of global terrorism at the top of the agenda of this year's G8 summit.

He also refused to criticise Algeria, saying the attack on the In Amenas gas complex had been an "extremely difficult" situation to deal with.

The priority now is to bring everyone involved in the incident home, the prime minister said.

Separately, Foreign Secretary William Hague confirmed that 22 British nationals survived the attack and were now back in Britain.

Cameron said: "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 this vicious and cowardly attack.

"And I would also say that when you're dealing with a terrorist incident on this scale, with up to 30 terrorists, it is extremely difficult to respond and to get this right in every respect."

Cameron said the threat in north Africa was from "an extremist Islamist violent Al-Qaeda-linked terrorist group", which he likened to militants in Pakistan and Afghanistan.

"It is similar because it is linked to Al-Qaeda, it wants to destroy our way of life, it believes in killing as many people as it can," Cameron said.

He added: "This is a stark reminder, once again, of the threat we face from terrorism the world over."

And he said: "This is a global threat and it will require a global response. It will require a response that is about years, even decades, rather than months.