Britain is braced for news of more casualties in the Algeria hostage crisis amid confusion over the death toll after a military rescue bid.
Prime Minister David Cameron warned of "further bad news" as he put off a high-profile speech on Europe to stay in London and oversee developments.
The Algerian authorities announced there had been "some deaths and injuries" in the operation to free foreign workers from Islamist militants.
One report on Algerian state television said two British workers were among four foreigners killed at the remote desert gas plant at In Amenas, which is jointly operated by BP.
Mr Cameron is to chair a meeting of the Cobra emergency committee as efforts continue to establish the full scale of the bloodshed. Foreign Secretary William Hague has cut short a visit to Australia to return to the UK and there is expected to be a ministerial statement to the Commons.
The Algerian rescue effort was launched early on Thursday without consultation with the UK, to the dismay of Number 10. Mr Cameron was informed that it was under way when he telephoned his Algerian counterpart on Thursday morning despite having earlier asked to be kept fully updated. Offers of British help had been declined.
Algerian communications minister Mohamed Said Belaid said the military operation succeeded in "neutralising a large number of terrorists and freeing a large number of hostages". "But unfortunately, we are sorry to say, there were some deaths and injuries," he said. "We do not yet have a definitive figure. As soon as we have it, we will make it public."
One British citizen had already been confirmed dead earlier in the hostage incident in which several others are known to be caught up. Earlier reports said that between six and 35 hostages and eight and 15 rebels had been killed.
Mr Cameron cancelled his long-awaited Europe speech which he had been due to deliver in the Netherlands today after a second call to Algerian premier Abdelmalek Sellal. Downing Street said he "emphasised the continuing need for the Algerian security forces to do everything they could to safeguard hostages".
Speaking afterwards he said: "We should be prepared for the possibility of further bad news, very difficult news, in this extremely difficult situation."