HERE is how news of the terrorist attack on the Algerian oil facility unfolded:
Wednesday January 16, 2am
Heavily armed gunmen in vehicles storm the energy site in In Amenas, in the east of the African country, taking a group of international workers hostage.
Unconfirmed reports say British workers are among the hostages, alongside a 36-year-old Irishman, Japanese, Norwegian and French nationals.
Tanaiste Eamon Gilmore calls for his compatriot's immediate release.
Officials at the UK Foreign Office say they are "urgently" investigating whether any British nationals were involved.
A BP spokesman confirms a "security incident" at the facility, activates its emergency response system and sets up a helpline for relatives.
Downing Street officials confirm British Prime Minister David Cameron will chair a meeting of the Government's crisis committee Cobra later in the day.
British Foreign Office confirms that Britons are caught up in the attack.
Officials say no family hotline has been set up for the incident because the number of British nationals involved is "very small".
Reports from Algeria's official news agency APS, attributed to provincial authorities, say one British and one French national were killed in the attack.
After a 45-minute meeting of Whitehall's Cobra emergency committee, Mr Cameron's official spokesman say he cannot confirm that a Briton has died but says "several" UK nationals are involved.
He says Mr Cameron will speak to his Algerian counterpart Abdelmalek Sellal later today.
Downing Street says it is "too early to speculate" on the motive for the attack, or whether it is linked to French support - backed by the UK - for Mali's government in its fight against Islamist forces which have occupied the north of the country.
It is reported that a news agency in the Saharan state of Mauritania was contacted by the militant group Katibat Moulathamine - "The Masked Ones" - with a claim that today's attack was carried out by one of its affiliates, identified as "Those who sign their names in blood".
A spokesman for the Katibat tells the Sahara Media Agency that 41 Westerners of nine or 10 nationalities were taken hostage, including seven Americans.
Five foreigners were being held in a factory, while 36 others were in living quarters at the plant, claims the spokesman, who says the action was carried out in retaliation for Algeria allowing France to use its airspace to carry out raids on northern Mali.