The remote desert gas facility is not far from the Libyan border
The hostage crisis at a gas facility in the Algerian desert where Islamist militants were holding foreign hostages has yet to be resolved, the UK says.
The Foreign Office said the "terrorist incident" near In Amenas was "ongoing".
Algerian state media said four hostages and several militants were killed when troops backed by helicopters attempted to free them on Thursday afternoon, reports the BBC.
The militants had claimed to be holding 41 foreigners. At least four were freed but the fate of many others is unknown.
Algeria has yet to give precise casualty figures from the rescue attempt.
The state-run APS news agency cited 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.
A spokesman for the militants told the Mauritanian ANI news agency that 35 hostages and 15 militants had been killed by helicopter gunfire in Thursday's operation.
On Friday morning, the UK Foreign Office said it believed the incident was ongoing, but the picture is confusing, reports BBC security correspondent Gordon Corera.
Norwegian Foreign Minister Espen Barth Eide told the BBC: "Parts of the plant are under Algerian authorities' control, and other parts are not. This information is changing by the hour."
The indications on Thursday night were that the military element of the operation had concluded, and that it had moved to the search phase.
But there are now some unconfirmed reports that a small group of militants and hostages remain, possibly near the gas compressor at the main gas plant, our correspondent adds.
It had always been reported that hostages were being held at different locations.
UK government sources said they were trying to establish the fate of as many as 20 British people and were bracing themselves for multiple casualties.
Japanese officials were meanwhile cited as saying by the Kyodo news agency that at least 14 Japanese nationals were still missing. At least three managed to escape.
Japan's Chief Cabinet Secretary, Yoshihide Suga, expressed "deep regret" at the actions of the Algerian security forces and its foreign ministry summoned the Algerian ambassador.
Norwegian Prime Minister Jens Stoltenberg said the Algerian officials had said "they felt they had no choice but to go in".
Norway said eight of its nationals were currently unaccounted for. One is being treated at a hospital in In Amenas, while four escaped unharmed.
French Interior Minister Manuel Valls said two French workers were safe. It was unclear if another two were involved, he added.
The Irish government confirmed that one of its citizens was free. Five Americans had survived and left the country, US officials told ABC News.
APS said about 600 local workers had been freed in the raid, but many of those were reportedly allowed to leave on Wednesday by the militants.