Algerian commandos are pursuing Islamic militants who are believed to be still holding a number of foreign hostages at a remote gas facility in Algeria.
"The operation is still going on," British Prime Minister David Cameron said, 36 hours after Algerian special forces stormed the complex, in which Algerian authorities said four foreign workers and 18 militants have been killed.
Making a statement in Parliament on the deadly rescue bid, Cameron said, "Hostages are still being held in the complex".
BBC said the militants had claimed that they were holding 41 foreigners. At least four of them have been freed but the fate of many others was still unknown.
British Petroleum, which runs the complex in collaboration with the Algerian government, said it had evacuated hundreds of workers from international oil companies and many more would follow.
Cameron told Parliament that the Algerian Army was still pursuing the terrorists and searching for hostages at the site. He said only the first part of the operation was complete.
"This is a large and complex site. They are still pursuing terrorists and possibly some of the hostages in other areas of the site," he said.
The militants had claimed firing by Algerian helicopters killed 35 hostages and 15 kidnappers, a figure disputed by the Algerian authorities.
The state-run APS news agency quoted local officials as saying two Britons and two Filipinos were killed.
Two others, a Briton and an Algerian, died on Wednesday when the militants ambushed a bus that was taking foreign workers at the facility to the local airport, BBC said.
The British prime minister said 30 Britons were still unaccounted for but the number is now considerably reduced.
Giving details of the attack, Cameron said the Islamist militants first attacked two buses en route to the airfield at the remote desert complex on Wednesday, killing two men.
Later, he said, they attacked a residential compound at the plant before turning to the gas facility itself. The plant is located just few kms from the Libyan border.
Among the foreign workers who were unaccounted for were Americans, Britons, French, Norwegians, Malaysians, Japanese and Filipinos.
French Interior Minister Manuel Valls described the situation as "still murky".
United States Defence Secretary Leon Panetta, who is on a visit to London, said that Washington and London were working closely with the Algerian government to assess what is happening on the ground.
Speaking today at the King's College London, Panetta said the US is working round the clock to ensure the safe return of its citizens.
"We deeply regret the actions taken by the Algerian military," Yoshihide Suga, Japanese chief cabinet secretary said, even as the Japanese foreign ministry summoned the Algerian ambassador.
Japanese officials were quoted as saying by the Kyodo news agency that at least 14 Japanese nationals were still missing while at least three had managed to escape.
The UK, Japan and US said they had not been informed in advance about the military assault.
Algerian Communications Minister Mohand Said Oubelaid said, "Those who think we will negotiate with terrorists are delusional."
Cameron said the Algerian prime minister had told him that commanders had "judged there to be an immediate threat to the lives of the hostages and had felt obliged to respond".