Foreign Secretary William Hague has accused the terrorists who killed a British national in the Algerian hostage crisis of "cold-blooded murder".

Several British workers are still in the gas field complex which was stormed by heavily armed militants on Wednesday and one other hostage has reportedly been killed.

An Islamist group claimed it was holding 41 Westerners - including seven Americans - in retaliation for the French military intervention against al Qaida-backed rebels in neighbouring Mali. The Irish government said a 36-year-old Irishman was among the hostages. He was believed to be unharmed.

Mr Hague described the siege as a "dangerous and rapidly developing situation" and has despatched a specialist response team to Algeria to back up Britain's embassy staff. He told the BBC: "This is an absolute tragedy, of course. In this dangerous and rapidly developing situation the next of kin have been informed. The Government's Cobra emergency system is in full operation. I have spoken to our ambassador in Algeria and despatched a rapid deployment team to Algeria to strengthen our embassy there and help them in their work. Excuses being used by terrorists and murderers who are involved - there is no excuse for such behaviour, whatever excuse they may claim. It is absolutely unacceptable, of course. It is, in this case, the cold-blooded murder of people going about their business. So there is no excuse, whether it be connected to Libya, Mali or anywhere else."

The Algerian government is in talks with the United States and France over the possibility of bringing in an international force to break the siege, unnamed security officials have claimed. Algerian forces have surrounded the In Amenas gas facility close to the Libyan border and tribal elders with links to Islamist militants have been contacted in an attempt to help negotiate an end to the stand-off, according to unconfirmed reports

David Cameron, who chaired a ministerial Cobra meeting on Wednesday, spoke on Wednesday night to his Algerian counterpart Abdelmalek Sellal. The Prime Minister "expressed his sympathy and support" and the two leaders agreed to keep in touch as the situation progresses, Downing Street said. The Prime Minister will chair a Cobra meeting on Thursday morning, Cabinet Office officials confirmed.

The Algerian interior ministry said the attack began when three vehicles carrying heavily armed militants ambushed a bus carrying employees from the gas plant to the nearby airport. Initially they were driven off, but they then headed for the main complex.

The militant group Katibat Moulathamine - "The Masked Ones" - later contacted a news agency in the Saharan state of Mauritania to claim the raid was carried out by an affiliate group, identified as "Those who sign their names in blood". A spokesman for the Katibat told the Sahara Media Agency the action was carried out in retaliation for Algeria allowing France to use its air space to carry out raids on northern Mali. Britain has provided two RAF C-17 transport aircraft to support the operation as well as offering to share intelligence with Paris. The attack happened as EU foreign ministers prepare to meet on Friday in Brussels to discuss plans to send a 400-strong military training mission to Mali.

The In Amenas facility is jointly operated by BP, Norwegian firm Statoil and Algerian state oil company Sonatrach. Statoil said it had 20 employees in the facility. Norwegian newspaper Bergens Tidende reported that a 55-year-old Norwegian working on the site called his wife to say he was among the hostages. The Tokyo government said Japanese employees working for a company which supplies services to the site might also have been kidnapped.

BP said armed groups were still on site holding a number of its staff, and described the situation as "unresolved and fragile". The company said it would not be releasing any information on those being held to "ensure we do not risk compromising the safety of these staff in any way". It added: "We are in contact with the families of those BP staff, offering them support and information. We will remain in frequent contact with the families to provide support and update them on developments. BP is in regular contact with the Algerian authorities, with our partners at Statoil and with other companies involved in the situation." BP chief executive Bob Dudley said: "BP's overriding priority is to do all we can to ensure the safety of our staff and to support their families during this anguishing time. All our efforts are focused on supporting the authorities to secure a peaceful resolution of the situation and the safe return of our colleagues, and all other workers being detained."