Britain was not informed in advance of the Algerian decision to stage a military intervention in the hostage crisis, Downing Street has said.
David Cameron was told that an operation was under way only when he telephoned the Algerian prime minister at 11.30am, the Prime Minister's official spokesman said.
The Prime Minister made clear that he would have preferred to be informed in advance, but the Algerians said they had had to act "immediately".
"The Prime Minister is extremely concerned. It is a very grave and serious situation," the spokesman said.
The Government has confirmed that there are "several" British nationals among the foreign hostages held by Islamist militants at the gas plant at In Amenas, deep in the Algerian desert.
The spokesman would not be drawn on reports that between six and 35 hostages and between eight and 15 of the rebels had been killed in the fighting.
"We are liaising closely with the Algerian authorities and other international partners to establish precisely the situation on the ground," the spokesman said.
"The Algerian prime minister explained that the situation is extremely fast-moving and that the Algerian government's judgment was that they needed to act immediately."
Mr Cameron was said to have made clear that he would prefer to have been informed in advance, both when he spoke to prime minister Abdelmalek Sellal yesterday about the situation and again in their telephone call today.
The spokesman refused to comment on the operation, but said the Government would like to have seen the crisis resolved "as peacefully as possible". He confirmed that while Britain offered assistance, the Algerians had not asked for any help.