Islamist militants claim to have kidnapped up to 41 foreigners after attacking a gas plant in eastern Algeria.
150 Algerian employees of a catering company are also being held, according to reports quoting the head of CIS Catering.
Three people are reported killed: one Briton, a French national and an Algerian security guard.
The hostages are believed to have several nationalities and include Britons, French, Americans and Japanese. Norway says 13 of its citizens are held.
The attackers claim they are linked to al Qaeda and came from Mali. The plant near In Amenas is jointly run by BP, the Norwegian company Statoil and the state Algerian firm Sonatrach.
BP has said it believes the gas field is shut following the attack. The company said the field normally produces over a tenth of Algeria’s overall gas output.
Al Qaeda in the Islamic Maghreb has blamed Algeria’s decision to allow France to use its airspace for its intervention in Mali.
In a video from earlier this week a spokesman warned France to stop its hostilities or it would be “digging the graves of its own sons”.
One employee at the site is quoted as saying the hostage-takers are demanding the release of 100 prisoners held in Algeria.
On Monday the Algerian army said it killed three terrorists and captured two others as they tried to enter the country.
The Algerian Interior Minister Daho Ould Kablia has said the authorities “will not respond to the terrorists’ demands” and has ruled out negotiations.