"Several" British nationals have been caught up in a terrorist attack on an oil facility in Algeria, Downing Street have said.
UK sources were unable to confirm local reports that one Briton has died in the incident, at a BP oilfield near the border with Libya.
Prime Minister David Cameron chaired a 45-minute meeting of Whitehall's Cobra emergency committee today, at which ministers were updated on the developing situation.
Speaking after the meeting, Mr Cameron's official spokesman said: "The ongoing incident has involved various nationalities, including several British nationals.
An Islamist militant group has claimed to have kidnapped up to 41 foreigners - including seven Americans and a Frenchman - in a dawn raid on the facility in retaliation for France's intervention in neighbouring Mali.
A 36-year-old Irishman is also among the group of workers who were taken hostage in the early hours by militant Islamist but is believed to be unharmed.
Ireland's Tanaiste (deputy prime minister) Eamon Gilmore has called for the immediate release of the Irish hostage.
"The Government stands ready to use all the resources available to us to ensure that our citizen is released as soon as possible," he said.
Heavily armed gunman in vehicles are reported to have stormed the energy site in In Amenas, in the east of the African country, at around 2am.
It is understood that some of the captive workers are being kept in their own living quarters at the compound and were being allowed access to telephone and email.
The natural gas complex is a joint venture of BP, Norway's Statoil and the Algerian Sonatrach company.