One is being treated at a hospital in In Amenas, while four escaped unharmed.
No foreign reporters are thought to have been given access to the In Amenas plant.
Forty-eight foreign workers were killed following a siege at the In Amenas plant on 16 January.
The precise number of casualties, evacuees and people missing from the In Amenas complex remains unclear.
And now in In Amenas it appears a Canadian citizen may have been one of the attackers.
In Amenas harbors nearly 50 oil and natural-gas fields, linked to the northern coast through several pipelines.
Yet companies are only expected to pull out if the In Amenas attack becomes part of a pattern.
Mr Barlow was a system supervisor for BP at the In Amenas plant.
The In Amenas field is operated as a joint-venture between Algeria's Sonatrach, Norwegian operator Statoil and UK energy giant BP.
An Algerian military operation is under way at the facility near In Amenas, the UK Foreign Office has confirmed.
BBC: Algeria siege: Alex Salmond confirms Scots among hostages
The crisis at the remote In Amenas desert gas facility began on Wednesday when militants attacked two buses carrying foreign workers.
Some foreign hostages were killed when forces raided the southeast Algerian natural-gas facility, called In Amenas, an Algerian government official confirmed.
Americans were among the hostages at the In Amenas facility in Algeria, White House spokesman Jay Carney said without specifying how many.
The UK ambassador is in nearby In Amenas to give consular support.
State-run APS news agency said those freed at the In Amenas installation included 573 Algerians and "around 100" out of 132 foreign workers.
The In Amenas gas field and its facilities are operated by a joint venture of the Algerian national oil company Sonatrach, BP and Statoil.
Western intelligence officials believe the camps may have been used by those who attacked the In Amenas gas facility in January, according to the sources.
The crisis started after hostage-takers attacked a gas plant at Tigantourine, near In Amenas, in the Sahara desert on Wednesday 16 January, Algerian officials said.
Mr John, a former Norwich School pupil, was among six UK nationals killed after the attack at the In Amenas gas plant, on 16 January.
The Northern Ireland man is being held at a gas facility at In Amenas, in the east of Algeria, along with about 20 other foreign nationals.
Sebastian John, 26, of Surrey, was one of six Britons believed to have been killed at the In Amenas gas plant after extremists stormed it last month.
The Tigantourine facility is about 40km (25 miles) south-west of In Amenas, which is close to the Libyan border and about 1, 300km (800 miles) south-east of Algiers.
The In Amenas gas field is situated at Tigantourine, about 40km (25 miles) south-west of the town of In Amenas and 1, 300km (800 miles) south-east of Algiers.
The British ambassador, Martyn Roper, along with a small consular and political team, travelled to In Amenas, the town close to the plant, to give consular support.
The agency said residents of the nearby town of In Amenas were staying indoors, amid rumours that the army operation to end the siege was not over.
BBC: Algeria crisis: Hostage-takers 'taken alive' at gas plant
Algerian security officials say one person was killed and others abducted in an attack on a gas facility at In Amenas in the east of the country.
What made the In Amenas attack so violent and lethal may turn out to be due to the very specific nature of how events unfolded on the ground.
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The Tigantourine gas facility is about 40km (25 miles) south-west of In Amenas, which is close to the Libyan border and about 1, 300km (800 miles) south-east of Algiers.
One thing is certain, though - after In Amenas, the Algerian government will strengthen its security apparatus in the Sahara, as beneath its sand lies much of Algeria's wealth.
"We have been informed by the UK and Algerian governments that the Algerian army is attempting to take control of the In Amenas site, " BP said in a statement.
BBC: Algeria siege: 'Victims' as army tries to free hostages
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