An Irishman is among about 20 workers kidnapped by a group claiming to be al Qaida at a Swedish oil plant in Algeria.
It is understood the 36-year-old married man, living in Northern Ireland, was among British, Japanese, Norwegian and French nationals who were taken captive on Wednesday morning.
They are being kept in a compound by their abductors and it is believed there have been injuries among the workers.
Ireland's Department of Foreign Affairs refused to disclose the location of the incident while Tanaiste Eamon Gilmore called for the man's immediate release. He 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."
The Department of Foreign Affairs said it was providing consular assistance to the family and was in close contact with its international partners and a wide range of other contacts in order to establish the facts of the situation. A spokesman added: "At this stage, the identity and motives of the kidnappers is unknown."
Officials at the UK Foreign Office said they were "urgently" investigating whether any British nationals were involved.
Islamist militants are understood to have kidnapped international members of staff from a BP site near the border with Libya.
A Foreign Office spokesman said: "A terrorist incident is ongoing near the town of In Amenas at an oil installation near the Algerian border with Libya. The British embassy in Algiers is aware of the incident and is liaising with local authorities as appropriate. We are aware that there are British nationals working in the area. We cannot at this stage confirm whether any are involved in the incident. We are urgently seeking clarification from the oil companies that are working in the area."
A BP spokesman said: "We can confirm that there has been a security incident this morning at the In Amenas gas field in the eastern central region of Algeria. The In Amenas field is operated by a joint venture of which BP is a member. We have no more confirmed details at this time. We will provide further updates. Algerian authorities are engaged with the incident; UK authorities have also been advised. BP has activated its emergency response system and is setting up a helpline for relatives."
UK Prime Minister David Cameron will chair a meeting of the Government's crisis committee Cobra later on the incident, Downing Street officials confirmed.