Fears are mounting over the fate of western hostages seized by Islamic militants in Algeria amid reports that as many as 35 had been killed during fierce fighting.
The militants reportedly claimed the hostages were killed after Algerian helicopters began strafing the plant. Fifteen kidnappers were also reportedly killed.
A number of Britons and an Irishman are known to be among the hostages. One Briton was thought to have been killed earlier when the militants attacked the gas field.
Earlier, an Algerian official said 20 people, including Europeans and Americans, had got away from the kidnappers, who had claimed they were holding 41 foreigners.
Britain is allowing the Algerian government to take the lead in the crisis and has received no requests for support, Downing Street said.
A spokesman for the militants - said to be speaking by telephone from the gas complex at In Amenas - told the ANI news agency in neighbouring Mauritania the hostages were being moved in oil company vehicles to another part of the site when the air attack happened.
Earlier, one hostage, identified as a Briton, was quoted calling for negotiations to "spare any loss of life". He said: "We are receiving care and good treatment from the kidnappers. The (Algerian) army did not withdraw and they are firing at the camp."
Another hostage said they were being forced to wear explosive belts and the heavily armed gunmen were threatening to blow up the base if the Algerian army stormed it.
Speaking before the latest reports emerged, Scotland's First Minister Alex Salmond told MSPs that a number of Scots were among those being held.
Prime Minister David Cameron's spokesman, also speaking earlier, said the situation was "very serious and dangerous" but the Government was "working through" the Algerians and BP, which part-controls the facility that was stormed. "Our focus is on working through the Algerian government and the company," the Downing Street spokesman said.