Several British nationals have been caught up in a terrorist attack on an oil facility in Algeria, Downing Street has 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.
An Islamist militant group has claimed to have kidnapped up to 41 foreigners - including seven Americans - in a dawn raid on the facility in retaliation for France's intervention in neighbouring Mali.
But the situation on the ground remains unclear and it is thought that authorities fear the number involved may be even higher.
Prime Minister David Cameron chaired a 45-minute meeting of Whitehall's Cobra emergency committee, 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. We are working with BP to support the families of staff and provide consular assistance."
Heavily armed gunmen in vehicles are reported to have stormed the energy site in In Amenas, in the east of the African country, at around 2am, taking a group of up to 20 international workers hostage. It is understood at least some of the captive workers were being kept in their own living quarters at the compound and were being allowed access to telephone and email.
Mr Cameron's spokesman declined to give precise details of the numbers of Britons believed to be involved in what he said was a "highly sensitive" situation, or whether any of them have been kidnapped or injured. He was unable to confirm reports from Algeria's official news agency APS, attributed to provincial authorities, that a Briton was among two people killed in the attack, with the other fatality understood to be French.
Mr Cameron will speak to his Algerian counterpart Abdelmalek Sellal later. Downing Street said that it was "too early to speculate" on the motive for the attack, or whether it was 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.
The Foreign Office said UK workers are involved in the incident but officials could not confirm reports of the killing. A 36-year-old Irishman is among the group of workers taken hostage in the early hours by a group claiming to be al Qaida but is believed to be unharmed.