A small group of jihadists was last night still controlling a part of the In Amenas plant while surrounded by Algerian forces, after a fierce battle left some 30 hostages dead.
Reports said at least one American had died in the Algerian hostage stand-off last night.
Of 650 hostages freed, 100 were foreigners, Government sources said.
One of the survivors, believed to be Iain Strachan from Howwood, Renfrewshire, said: "I am very relieved to be out. We still don't know what's happening back on site. As much as we are glad to be out, our thoughts are with colleagues that are still there at the moment."
Unofficial reports suggest between seven and 10 attackers armed with explosives were still in the gas plant's machine room.
Prime Minister David Cameron expressed his disgust and condemnation at the seige and said the terrorists responsible for the brutal and savage attack would be hunted down.
He said he was told by his Algerian counterpart, Abdelmalek Sellal, that troops were still pursuing terrorists and possibly some of the hostages in other areas of the site.
He added: "The Algerian Prime Minister has [told me] they are now looking at all possible routes to resolve this crisis. 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."
As Algeria faced criticism for launching the military assault without informing other governments, the state press agency said the Algerian Government felt the actions of the army avoided a real disaster.
It said a preliminary assessment showed 12 foreign and Algerian workers had been killed since the start of the army operation.
The agency said the army was still trying to reach a peaceful settlement before "neutralising the terrorist group that is entrenched in the refinery and releasing a group of hostages still being held".
US Secretary of State Hillary Clinton, who had spoken to Mr Sellal, said hostages remained in danger and warned him utmost care must be taken to preserve innocent life.
She added: "We remain deeply concerned about those who remain in danger."
First Minister Alex Salmond said all Scottish hostages had been freed and were safe and well. At least three Scots are known to have been caught up in the seige and released.
They include 29-year-old father-of-one Mark Grant from Grangemouth and a father-of-two from Fife who now lives in South Africa.
Mr Strachan, along with four other freed British hostages, spoke on Algerian TV. The men praised the army's role in their release in the wake of growing concern that Algeria acted without discussion with affected states.
Another Briton, identified as Darren Matthews, said: "Very relieved to be going back home to see friends and family... I feel safe at the moment, but I won't feel 100% happy until I'm back in the UK."
Another said: "I think they did a fantastic job. I was very impressed with the Algerian army. It was a very exciting episode. I feel sorry for anybody who has been hurt but, other than that, I enjoyed it."
The Algerian Government claims to have killed 18 of the Signers in Blood militant group, which said it carried out the attack.
There were reports surviving terrorists had been driven out of the gas field's living quarters, where they had taken hundreds of hostages, and into the plant itself.
A spokesman for the group thought to be behind the attack said it would carry out further operations. The militant group has reportedly offered to release an undisclosed number of American hostages in exchange for Sheikh Omar Abdel Rahman, who orchestrated the 1993 World Trade Centre bombing, and Aafia Siddiqui, a Pakistani woman who is jailed in the US on terrorism charges.
The US State Department said it would not negotiate with terrorists.
The terrorists have said the raid
was carried out because of Algeria's decision to allow France to use its airspace for attacks against Islamists in Mali, where French forces have been in action against militants since last week.
US Defence Secretary Leon Panetta said the US was working with Britain and other allies to free hostages. He did not say whether the US was considering armed intervention.
Meanwhile, Japanese officials said at least 14 Japanese nationals were still missing. At least three managed to escape.
Norway said eight of its nationals were unaccounted for, while five escaped.
French Interior Minister Manuel Valls said two French workers were safe.
One Irish citizen, an Austrian and five Americans have been freed.
French national Alexandre Berceaux said he had hidden under the bed in his room for 40 hours before being rescued.