A small group of jihadists have barricaded themselves into a corner of the In Amenas natural gas plant in eastern Algeria, where they are surrounded by government forces after a fierce battle that reportedly left 30 hostages dead and a similar number still unaccounted for.

Radio France reported that seven to 10 surviving members of the Signers in Blood faction who attacked the site on Wednesday had retreated into the machine room at the sprawling desert site, adding that they were armed with explosives. But it was unclear how many hostages they still had with them.

The authorities said that 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 that "quite significantly" fewer than 30 British citizens were still at risk. The Guardian understands the actual total of those unaccounted for is about 10.

Cameron 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.

"We are still dealing with a fluid and dangerous situation where a part of the terrorist threat has been eliminated in one part of the site, but there still remains a threat in another part," Cameron said.

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.

The surviving hostages described scenes of turmoil and terror. Some said a number of foreign contractors had been killed by the jihadists. An Algerian worker from the city of Ouargla who escaped when the army stormed the camp said: "There were so many deaths. I was shocked. I can't say the exact number. Foreign hostages died, Algerian hostages died and there were deaths among the terrorists."

Alexandre Berceaux, an employee of a French catering firm, told Europe 1 radio he had survived by staying in his room, away from other foreigners, hidden behind a barricade of wooden planks. Algerian workers kept him alive by bringing him food and water, and they eventually brought Algerian soldiers to his hideout.

"I was completely isolated ... I was afraid. I could see myself already ending up in a wooden box," Berceaux said in an interview from an Algerian military base.

The Algerian Press Service quoted a security official as saying: "[The army] is still trying to achieve a peaceful outcome before neutralising the terrorist group that is holed up in the [facility] and freeing a group of hostages still being held."

Japan said three of its nationals had escaped but 10 were still not accounted for. The US defence secretary, Leon Panetta, who met Cameron in London, said "somewhere in the vicinity" of seven or eight Americans had been taken hostage. US officials said one American, Frederick Buttaccio, a Texas resident, had died but said it was unclear how. They said US officials recovered Buttaccio's remains on Friday and notified his family.

The Norwegian government said one of its citizens escaped from the gas complex overnight and was recovering in hospital, but it did not know the fate of eight others.

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, on the border with Libya, because of the continuing fighting. Some British survivors of the ordeal were instead being flown out of the region through Hassi Messaoud.

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. In the team there were also officials, believed to be from the intelligence agencies, to debrief hostages before they returned to the UK.

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, and reports from the area said the surviving guerillas had been driven out of the gas field's living quarters, where they had taken hundreds of hostages, and into the gas plant itself. The pumps have been turned off to avert 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 defended their actions, saying the militants were attempting to flee the scene with the hostages, who were believed to be in imminent jeopardy.

The government in Algiers issued a statement saying: "This operation saved hundreds of hostages' lives and averted a disaster at the gas installations."

However, the Mauritanian news agency ANI, which has stayed in contact with the militant group throughout the siege, quoted a source in its ranks as saying they had not intended to leave the gas field but were moving a group of hostages from one part of the complex to another when Algerian army helicopter gunships opened fire on the its vehicles ferrying the hostages and their captors.

ANI said the militants were threatening more attacks on oil and gas installations in Algeria. A spokesman for the group called on Algerians to "keep away from the installations of foreign companies, because we will suddenly attack where no one would expect it".

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 interior minister, Manuel Valls, said he "cautioned prudence against criticism" of the Algerian operation and recognised Algeria's fight against terrorism alongside France.

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 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.

Japan's prime minister, Shinzo Abe, cut short a three-nation visit to south-east Asia to return to Tokyo to oversee Japan's response to the crisis. Abe, who was due to make a foreign policy speech in Indonesia on Friday evening

, was expected to return to the Japanese capital early on Saturday.

There was irritation, bordering on anger, that the Algerian government had not notified Japan of the rescue attempt. Tokyo was told of the military operation by Britain's ambassador to Algeria, local reports said.

Abe told reporters he had asked the Algerian prime minister in a phone call to refrain from any moves that could threaten the safety of the hostages. Japan's main government spokesman, Yoshihide Suga, described the rescue attempt as regrettable, amid unconfirmed reports that two Japanese nationals were among those who died during the assault.The Japanese services contractor operating at the Algerian gas field, JGC, said that of the 61 non-Japanese it employed at the facility, 10 had been accounted for. JGC's president and other company executives were due to leave for Algeria on Friday.