A deadly Algerian military raid to free hostages and wipe out their Islamist militant captors moved closer to the heart of the natural gas complex today, the government news service said.
A total of 18 militants were killed and the plant's living quarters were secured, security officials told the government news agency.
Dozens of energy workers remained unaccounted for after the Algerian military's initial claim that the assault at the remote desert facility was over late yesterday.
Algeria's government has kept a tight grip on information, but it was clear that the militant assault that began on Wednesday has killed at least six people from the factory - and perhaps many more.
Three Britons are reported to be among the dead.
Workers kidnapped by the militants came from around the world - Americans, Britons, French, Norwegians, Romanians, Malaysians, Japanese, Algerians. Leaders today expressed strong concerns about how Algeria was handing the situation and its apparent reluctance to communicate.
Terrorised hostages from Ireland and Norway trickled out of the Ain Amenas plant, 1,300 kms south of Algiers, the capital. BP, which jointly operates the plant, said it had begun to evacuate employees from Algeria.
British Prime Minister David Cameron told MPs the situation remained fluid and dangerous, saying "part of the threat has been eliminated in one part of the site, a threat still remains in another part".
Algeria's army-dominated government, hardened by decades of fighting Islamist militants, shrugged aside foreign offers of help and drove ahead alone.
Today Algeria's ambassador to Japan was summoned and told that Japan demanded that Algeria prioritise hostages' lives and co-operate more closely.
The US government sent an unmanned surveillance drone to the BP-operated site, near the border with Libya, but it could do little more than watch the military intervention. British intelligence and security officials were on the ground in Algeria's capital but were not at the installation, said an official.
A US official said while some Americans escaped, other Americans were either still held or unaccounted for.
Algerian security forces moved in yesterday, first with helicopter fire and then special forces, according to officials. The government said it was forced to intervene because the militants were being stubborn and wanted to flee with the hostages.
Militants claimed 35 hostages died when the military helicopters opened fire as they were transporting hostages from the living quarters to the main factory area where other workers were being held.
The group - led by a Mali-based al Qaida offshoot known as the Masked Brigade - suffered losses in the military assault - but won a global audience.
Even violence-scarred Algerians were stunned by the brazen hostage-taking. Mass fighting in the 1990s had largely spared the lucrative oil and gas industry that gives Algeria its economic independence and regional weight.
The remote location is extremely hard to reach and was surrounded by Algerian security forces.
"An important number of hostages were freed and an important number of terrorists were eliminated, and we regret the few dead and wounded," Algeria's communications minister, Mohand Said Oubelaid, told national media adding that the "terrorists are multinational," with the goal of "destabilising Algeria, embroiling it in the Mali conflict and damaging its natural gas infrastructure."
The official news agency said four hostages were killed in yesterday's operation, two Britons and two Filipinos. Two others, a Briton and an Algerian, died on Wednesday in the initial militant ambush on a bus ferrying foreign workers to an airport. The APS news agency said six Algerians and seven foreigners were injured.
APS said some 600 local workers were safely freed in the raid - but many of those were reportedly released the day before by the militants themselves.
A spokesman for the Masked Brigade told the Nouakchott Information Agency in Mauritania that only seven hostages survived.
President Barack Obama and Mr Cameron spoke on the phone to share their confusion. White House Press Secretary Jay Carney said the Obama administration was "seeking clarity from the government of Algeria."
BP, the Norwegian company Statoil and the Algerian state oil company Sonatrach, operate the gas field and a Japanese company, JGC Corp, provides services for the facility.
Algerian Interior Minister Daho Ould Kabila said the 20-odd militants entered the country from nearby Libya in three vehicles, in an operation commanded by extremist mastermind Moktar Belmoktar, who is normally based in Mali.
The militants made it clear that their attack was in revenge for the French intervention against Islamists who have taken over large parts of neighbouring Mali.