Mokhtar Belmokhtar, the one-eyed jihadist thought to be behind the attack in Algeria.
Algerian Interior Minister Daho Ould Kablia said the kidnappers were led by Mokhtar Belmokhtar, a veteran Islamist guerrilla who fought Soviet forces in Afghanistan in the 1980s and had set up his own group in the Sahara after falling out with other local al Qaeda leaders.
A holy warrior-cum-smuggler dubbed "The Uncatchable" by French intelligence and "Mister Marlboro" by some locals for his illicit cigarette-running business, Belmokhtar's links to those who seized towns across northern Mali last year are unclear.
The hostage takers earlier allowed some prisoners to speak to the media, apparently to put pressure on Algerian forces not to storm the compound. An unidentified hostage who spoke to France 24 television said prisoners were forced to wear explosive belts and captors had threatened to blow up the plant.
A local source told Reuters the hostage takers had blown up a petrol filling station at the plant.
Two hostages, identified as British and Irish, spoke to Al Jazeera television and called on the Algerian army to withdraw from the area to avoid casualties.
"We are receiving care and good treatment from the kidnappers. The (Algerian) army did not withdraw and they are firing at the camp," the British man said. "There are around 150 Algerian hostages. We say to everybody that negotiations is a sign of strength and will spare many any loss of life."
Ireland said later that the Irish hostage was among those freed.
The precise number and nationalities of foreign hostages could not be confirmed, with some countries reluctant to release information that could be useful to the captors.
Britain said one of its citizens was killed in the initial storming on Wednesday and "a number" of others were held.
The militants said seven Americans were among their hostages, a figure U.S. officials said they could not confirm.
Norwegian oil company Statoil said nine of its Norwegian staff and three Algerian employees were captive.
Britain's BP said some of its staff were held but would not say how many or their nationalities. BP added that it had staged plans to bring a group of non-essential workers out of Algeria.
Japanese media said five workers from Japanese engineering firm JGC Corp were held, a number the company did not confirm.
Paris has not said whether any hostages were French.
Vienna said one hostage was Austrian, Dublin said one was Irish and Bucharest said an unspecified number were Romanian.
Spanish oil company Cepsa said it had begun to evacuate personnel from elsewhere in Algeria, an OPEC member.
Paris said the Algeria attack demonstrated it was right to intervene in Mali: "We have the flagrant proof that this problem goes beyond just the north of Mali," French ambassador to Mali Christian Rouyer told France Inter radio.
President Francois Hollande has received public backing from Western and African allies who fear that al Qaeda, flush with men and arms from the defeated forces of Libya's Muammar Gaddafi, is building a desert haven in Mali, a poor country helpless to combat fighters who seized its north last year.
However, there is also some concern in Washington and other capitals that the French action in Mali could provoke a backlash worse than the initial threat by militants in the remote Sahara.
The militants, communicating through established contacts with media in neighbouring Mauritania, said they had dozens of men armed with mortars and anti-aircraft missiles in the compound and had rigged it with explosives.
"We hold the Algerian government and the French government and the countries of the hostages fully responsible if our demands are not met, and it is up to them to stop the brutal aggression against our people in Mali," read one statement carried by Mauritanian media.
They condemned Algeria's secularist government for letting French warplanes fly over its territory to Mali and shutting its border to Malian refugees.
The attack in Algeria did not stop France from pressing on with its campaign in Mali. It said on Thursday it now had 1,400 troops on the ground in Mali, and combat was underway against the rebels that it first began targeting from the air last week.
"There was combat yesterday, on the ground and in the air. It happened overnight and is under way now," said Le Drian.
Residents said a column of about 30 French Sagaie armored vehicles set off on Wednesday toward rebel positions from the town of Niono, 300 km (190 miles) from the capital, Bamako.
The French action last week came as a surprise but received widespread international support in public. Neighboring African countries planning to provide ground troops for a U.N. force by September have said they will move faster to deploy them.
Nigeria, the strongest regional power, sent 162 soldiers on Thursday, the first of an anticipated 906.
"The whole world clearly needs to unite and do much more than is presently being done to contain terrorism, with its very negative impact on global peace and security," President Goodluck Jonathan said.
Germany, Britain and the Netherlands have offered transport aircraft to help ferry in African troops. Washington has said it is considering what support it can offer.
Many inhabitants of northern Mali have welcomed the French action, though some also fear being caught in the cross-fire.
The Mali rebels who seized Timbuktu and other oasis towns in northern Mali last year imposed Islamic law, including public amputations and beheadings that angered many locals.
A day after launching the campaign in Mali, Hollande also ordered a failed rescue in Somalia on Saturday to free a French hostage held by al Qaeda-linked al Shabaab militants since 2009.
Al Shabaab said on Thursday it had executed hostage Denis Allex. France said it believed he died in the rescue.