ALGIERS: About 60 foreign hostages are still unaccounted for three days into a bloody siege with militants at a gas plant deep in the Sahara, Algeria's state news service said Friday.    

The militants, meanwhile, offered to trade two American hostages for terror figures jailed in the United States, according to a statement received by a Mauritanian news site that often reports news from North African extremists.

It was the latest surprising development in a hostage drama that began Wednesday when militants seized hundreds of workers from 10 nations at Algeria's remote Ain Amenas natural gas plant.

Algerian forces retaliated on Thursday by storming the plant in an attempted rescue operation that killed at least four hostages and left leaders around the world expressing strong concerns about the hostages' safety.

Algerian special forces resumed negotiating Friday with the militants holed up in the refinery, according to the Algerian news service, which cited a security source.

The report said ''more than half of the 132 hostages'' had been freed in the first two days, but it could not account for the remainder, saying some could be hidden throughout the sprawling desert site.

Militants on Friday offered to trade two American hostages for two prominent terror figures jailed in the United States: the mastermind of the 1993 World Trade Center bombing and Pakistani neuroscientist Aafia Siddiqui who was convicted of shooting at two US soldiers in Afghanistan.

The offer, according to a Mauritanian news site that frequently broadcasts dispatches from groups linked to al-Qaida, came from Moktar Belmoktar, an extremist commander based in Mali who apparently masterminded the operation.

Algeria's government has kept a tight grip on information, but it was clear that the militant assault that began Wednesday with an attempted bus hijacking has killed at least six people from the plant, and perhaps many more.

Workers kidnapped by the militants came from around the world, Americans, Britons, French, Norwegians, Romanians, Malaysians, Japanese, Algerians.

Leaders on Friday expressed strong concerns about how Algeria was handing the situation and its apparent reluctance to communicate.

British Prime Minister David Cameron went before the House of Commons on Friday to provide an update, seeming frustrated that Britain was not told about the military operation despite having ''urged we be consulted.''

Terrorized hostages from Ireland and Norway trickled out of the Ain Amenas plant, 800 miles (1,300 kilometers) south of Algiers, the capital. BP, which jointly operates the plant, said it had begun to evacuate employees from Algeria.

''This is a large and complex site and they are still pursuing terrorists and possibly some of the hostages,'' Cameron said. He told lawmakers the situation remained fluid and dangerous, saying ''part of the threat has been eliminated in one part of the site, a threat still remains in another part.''

Algeria's army-dominated government, hardened by decades of fighting Islamic militants, shrugged aside foreign offers of help and drove ahead alone.

The US government sent an unarmed surveillance drone to the BP-operated site, near the border with Libya, but it could do little more than watch Thursday's military intervention.

British intelligence and security officials were on the ground in Algeria's capital but were not at the installation, said a British official who spoke on condition of anonymity because he was not authorized to speak to reporters.

A US official said while some Americans escaped, other Americans were either still held or unaccounted for.