Prime Minister David Cameron has said the country should be "prepared for the possibility of further bad news" in the hostage crisis in Algeria.
With a confused picture of what is happening on the ground following an Algerian military operation, Mr Cameron has postponed his speech on Europe in the Netherlands that was due on Friday to stay in Downing Street.
One British citizen is known to have died in the crisis and several others have been caught up in it.
Mr Cameron said: "It's a fluid situation, it's ongoing, it's very uncertain.
"We should be prepared for the possibility of further bad news, very difficult news, in this extremely difficult situation."
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.