An Irishman is among several workers taken hostage by a group claiming to be al Qaida at a gas field in Algeria.
It is reported that two people have been killed in the ongoing incident, but it is understood the 36-year-old married man, from Northern Ireland, is unharmed.
Officials at the Foreign Office said they were urgently investigating whether any British nationals were involved in the "terrorist incident" at the plant in In Amenas, in the east of the African country.
Prime Minister David Cameron will chair a meeting of the Government's crisis committee Cobra later on the incident near the border with Libya, Downing Street officials confirmed.
Ireland's deputy prime minister Eamon Gilmore has called for the immediate release of the Irishman. "The Government stands ready to use all the resources available to us to ensure that our citizen is released as soon as possible," he said.
Conflicting reports said there were between six and around 20 people being held at the gas facility since the Islamist militants seized control at around 2am.
It is understood at least some of the captive workers were being kept within their own living quarters at the compound and were being allowed access to telephone and email.
Oil giant BP confirmed there was a "security incident" at the In Amenas gas field, which is operated as a joint venture, involving BP, Norway's Statoil and Algerian energy company Sonatrach. A BP spokesman said: "BP has activated its emergency response system and is setting up a helpline for relatives."
A Foreign Office spokesman said the British embassy in Algiers was working with local authorities. Initial reports suggested British, Norwegian, Japanese and French nationals were among those taken hostage.
It was also reported that the Algerian army had surrounded militants and their hostages while negotiations were ongoing.