An Irishman is among a group of up to 40 workers taken hostage by Islamist militants at an oil plant in eastern Algeria.
It is understood the 36-year-old married man from Northern Ireland was among British, Japanese, Norwegian and French nationals who were taken captive early this morning.
One British national and one French national were also reported to have died in the attack.
Militants have demanded an end to French military operations against Islamists in northern Mali in return for the safety of the hostages, Mauritania's ANI news agency reported.
The statement sent to ANI by the group also said it was holding 40 hostages.
A spokesman for a group under the command of Mokhtar Belmokhtar, a veteran Saharan jihadist, had earlier said it was behind the hostage-taking.
It later claimed to have repelled an attempt by Algeria's army to enter the facility.
The natural gas complex in In Amenas is a joint venture of BP, Statoil and the Algerian Sonatrach company.
BP said that the militants were still occupying the siite, but it could not confirm whether there had been any casualties in the attack.
BP said: "The site was attacked and occupied by a group of unidentified armed people at about 5am UK time.
"Contact with the site is extremely difficult, but we understand that armed individuals are still occupying the In Amenas operations site."
The Department of Foreign Affairs is providing assistance to the family of the Irish citizen and has called for his immediate release.
It is in close contact with its international partners and a wide range of other contacts to establish the facts of the situation.
Tánaiste Eamon Gilmore said: "The Government stands ready to use all the resources available to us to ensure that our citizen is released as soon as possible.
"I would ask that the family be allowed privacy at this difficult time."