The foreign secretary has said the Algerian hostage crisis will remain the government's "top priority until every British national is accounted for".
William Hague's comment on Twitter came before he chaired another meeting of the Cobra emergency committee, as the standoff between the Algerian military and al-Qaida-linked jihadists at the In Amenas gas facility entered its fourth day.
The Guardian understands that about 10 Britons remain unaccounted for, of whom at least one may be held hostage by the militants at the industrial plant in the east of the country.
Radio France reported that seven to 10 surviving members of the Signers in Blood faction armed with explosives are holed up in the machine room of the sprawling desert site, making it difficult for Algerian special forces to intervene.
The army, which has taken control of the residential quarters at the plant, has surrounded the site, and helicopters are monitoring the area, Algerian state radio said on Saturday.
The militants have said they are holding seven foreign hostages, one Briton, two Americans, three Belgians and one Japanese. A total of about 30 foreigners remain unaccounted for.
Hague added: "My thoughts are also with the families of everyone affected, particularly those still waiting for news of their loved ones."
One Briton was among 12 Algerian and foreign workers reported killed when militants attacked the site on Wednesday.
The authorities said more than 570 Algerian workers had been freed following a military rescue mission on Thursday and that 100 of 132 foreign contractors taken hostage at the gas field had been rescued or had escaped.
David Cameron told the House of Commons on Friday that "quite significantly" fewer than 30 British citizens were still at risk.
The prime minister spoke twice to the Algerian prime minister, Abdelmalek Sellal, on Friday, to offer the help of UK special forces, and came away from his talks believing the Algerian government would not mount another unilateral all-out attack on the gas plant.
British sources said it was a relief that a line of communication appeared to have been opened between the hostage takers and the Algerian government.
Surviving hostages described scenes of turmoil and terror. Some said the jihadists had killed a number of foreign contractors.
A plane carrying a 15-strong rapid deployment team of British consular staff, foreign officials, police and Red Cross counsellors landed in the desert outpost of Hassi Messaoud on Friday afternoon, but was not given clearance to fly the remaining 300 miles south-east to In Amenas, because of the continuing fighting.
The Foreign Office said BP, a partner in the gas field, had taken the lead in organising the evacuation of rescued and escaped British workers but the government team was there as a contingency to offer support.
A US plane was sent to pick up freed American hostages and a US drone was reported to be flying over the gas field.
The Algerian government claimed to have killed 18 of the Signers in Blood militant group. The pumps have been turned off to prevent an explosion.
After sharp criticism from Japan for launching a military assault on the hostage-takers, and from the UK for not informing London before launching the operation, the Algerian authorities have defended their actions, saying the militants were attempting to flee the scene with the hostages, who were believed to be in imminent jeopardy.
France, which is dependent on the co-operation of Algeria for the smooth running of its operation against Islamists in Mali, was positive in its comments on the Algerian action.
The French president, François Hollande, said he had every confidence in the Algerian authorities to handle the situation, which he referred to as "unfolding in dramatic conditions".
Cameron said on Friday he would not hide his disappointment at the lack of Algerian consultation, but No 10 sources said the Algerians looked to have faced a well-organised terrorist assault that was probably planned before the French invasion of Mali.
