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

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 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 today 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 today 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 a Pakistani scientist convicted of shooting at two U.S. soldiers in Afghanistan.

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

Algerian security forces moved in Thursday, first with helicopter fire and then special forces, according to diplomats, a website close to the militants, and an Algerian security official. The government said it was forced to intervene because the militants were being stubborn and wanted to flee with the hostages.

Militants claimed 35 hostages died when the military helicopters opened fire as they were transporting hostages from the living quarters to the main factory area where other workers were being held.

The group — led by a Mali-based al-Qaida offshoot known as the Masked Brigade — made it clear that their attack was in revenge for the French intervention against Islamists who have taken over large parts of neighboring Mali.