Gunmen in an ongoing stand-off with Algerian forces at a gas compound have reportedly demanded safe passage, amid reports up to 25 foreign hostages are now free.
William Hague earlier condemned the killing of a Briton at the plant near the border with Libya as "cold-blooded murder".
The Briton and an Algerian were killed when around 20 attackers from an al Qaeda-backed group stormed the In Amenas facility and claimed to have taken 41 hostages.
Several British people were known to be among those taken captive. Scottish National Party leader Alex Salmond has confirmed they included Scottish people.
Various unconfirmed reports of hostages escaping or being freed from the base have emerged. In the latest, an Algerian security source said 25 people had been freed and that two of them were Japanese.
Six people were wounded in Wednesday's attack, which the group claims is a retaliation for the French military intervention against al Qaeda-backed rebels in neighbouring Mali.
The raid is believed to have been planned by Mokhtar Belmokhtar, an Islamist known as Mr Marlboro and The Uncatchable.
His group goes under various names including Khaled Abul Abbas Brigade, the Masked Ones and The Blood Battalion and is said to have links with al Qaeda in the Islamic Maghreb, a Mali-based militant group that wants to overthrow Algeria's government.
Mr Hague said he was sceptical the attack was a retaliation over the offensive against Islamist fighters in Mali because it would take longer to plan.
David Cameron earlier chaired emergency meeting on the crisis. Downing Street said it still believes the best approach is to work through the Algerian government and oil companies who share ownership of the base, such as BP.
The siege showed signs of "considerable" planning, Downing Street added.
The Prime Minister has said he will consider "any requests for assistance" from Algeria - but no such requests have been made so far, Sky's Tim Marshall said.
Algerian interior minister Dahou Ould Kablia has said his government would not negotiate with "terrorists".
An Algerian security official has said the government is in talks with the US and France over whether an international force could help.
Meanwhile, the wife of one Norwegian worker who escaped as gunmen launched their attack on a bus that was departing the site said he had been shot at.
Vigdis Sletten told Norway channel VGTV her husband, who is now being protected in a military camp, had said in a phone call: "Their guards returned the fire while they threw themselves on the floor in the bus.
"They escaped through the bus window and were taken to a military camp.
"I do not know if he is injured or not."
In its latest statement, BP said the situation remains "unresolved and fragile".
The Irish government has said a 36-year-old Irish national was among the hostages. He was believed to be unharmed.
The militant group Katibat Moulathamine - The Masked Ones - has claimed responsibility. A spokesman said the action was carried out in retaliation for Algeria allowing France to use its airspace to carry out raids on northern Mali, where France now has 1,400 troops on the ground.
Britain has provided two RAF C-17 transport aircraft to support the Mali operation as well as offering to share intelligence with Paris.