Seven hostages are said to have been killed by Islamist militants at the In Amenas facility, prompting Algerian forces to make a final assault, resulting in the deaths of 11 hostage takers.

(Read our full coverage on the crisis at In Amenas here.)

Sixteen other foreign hostages are reported to have survived in the final operation by forces which ended a deadly siege which began early on Wednesday morning.

UK Defence Secretary Philip Hammond confirmed the crisis was over and that lives had been lost, but the nationalities of the deceased were not revealed. UK Prime Minister later confirmed the end of the crisis following a conversation with his Algerian counterpart, Abdelmalek Sellal.

Hostages which had been held included Britons, Americans and Norwegians.

The BBC has reported that at least 19 hostages and 29 militants are now thought to have died since a dual attack on a crew bus and the gas facility on Wednesday. Earlier on Saturday, Algerian special forces are reported to have found 15 burned bodies at the plant.

BP said on Saturday afternoon that four of its staff remained unaccounted for with chief executive Bob Dudley fearing "one or more" deaths amongst this number.

Statoil had said six of its workers were also unaccounted for while Britain said that fewer than 10 Britons remained unaccounted for.

UK Foreign Secretary William Hague said on Saturday evening that five Britons and one UK resident had been killed or were unaccounted for following the final assault. One British national had already been confirmed as killed in the original attack on Wednesday.

Precise details of the final operation which brought the crisis to an end remain sketchy, but reports suggest the kidnappers had executed seven hostages prompting Algerian forces to take action to end the standoff. Other reports suggest there may have been a fire at the facility which hastened an end to the situation.

On Saturday evening Statoil chief executive Helge Lund said one more employee - a Norwegian - was safe out of the six remaining Statoil personnel who had remained unaccounted for since the beginning of the crisis. However, he said there is a "deep and growing concern" for the five, all of whom are believed to be Norwegian.

"The situation remains unresolved for five Statoil employees, and neither the Norwegian Ministry of Foreign Affairs nor Statoil have confirmed information about their whereabouts," Statoil said.

"We must prepare ourselves for the fact that it may take time before we get a response. We are facing difficult hours and days,” Lund said.

"We are working hard to provide information and convey assistance," he continued. "We want, and we must, keep hoping for more positive news from Algeria. However, we must be prepared to deal with bad news in the next few days.

“What has happened in Algeria is brutal, international terrorism of the worst kind. Despicable acts by ruthless terrorists have created an international tragedy."