UK Prime Minister David Cameron said questions would be asked about the Algerian response to the crisis
Three Britons are confirmed to have been killed and three more are believed to be dead after the Algeria hostage crisis, the prime minister has said.
One further UK resident is also thought to be dead, David Cameron said.
An unknown number of Britons have survived. Some returned to the UK overnight, the Foreign Office said.
Algerian troops raided the In Amenas gas facility on Saturday ending a four-day siege. At least 23 hostages died and Algeria says 32 militants died.
Mr Cameron said: "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 who now face life without these very precious loved ones."
The prime minister said questions would be asked about the Algerian response to the crisis, but he added: "The responsibility for these deaths lies squarely with the terrorists who launched this vicious and cowardly attack."