LONDON -- Amid the "serious" hostage crisis at the facility at the  Amenas gas plant in Algeria, BP (IW 1000/4) said Friday that a "small number" of its staff at the plant were unaccounted for, adding that it had evacuated hundreds of workers.

"There is a small number of BP employees at In Amenas whose current location and situation remain uncertain," the British energy giant said.

"BP is working with the Algerian government and authorities to confirm their status. We do not intend to publicly comment on details of the number, nationalities or identities of these staff.

"BP is in direct contact with the families of all of those staff affected, offering support and providing confirmed information as we can."

The British energy giant added: "The serious situation at the In Amenas site in eastern Algeria remains ongoing. The situation remains unclear and BP continues to seek updates from the authorities."

The London-listed company, which operates the site, said it had evacuated workers from several Algerian facilities including In Amenas, where dozens of foreigners were taken hostage by Islamic militants.

"Overnight, BP began the staged process of bringing non-essential workers out of Algeria as a precautionary and temporary measure," the statement said.

"Three flights left Algeria yesterday, carrying a total of 11 BP employees alongside several hundred staff from other companies."

It added: "A fourth plane is expected to transport further staff out of the country today and we will arrange further flights as necessary."

Algerian troops launched an air and ground assault on Thursday on the In Amenas gas complex that had been seized by the Islamists. The kidnappers have threatened to kill their captives.

Some Islamist hostage-takers were still holed up at a remote gas field in Algeria on Friday, a security source said, adding that their claim of dozens of people killed in an army rescue bid was "fantasy."

The Amenas gas field is a joint venture between BP, Norwegian group Statoil and Algerian state-owned oil firm Sonatrach, and is situated in the remote Sahara desert in eastern Algeria, not far from the Libyan border.

"Supporting our colleagues and their families at a time of such extreme concern is essential, and we are seeking to support them in every way we can," BP chief executive Bob Dudley said.

Copyright Agence France-Presse, 2013