Mokhtar Belmokhtar
Mokhtar Belmokhtar is said to have led the militants' raid on the In Amenas gas plant in Algeria (YouTube)

At least 15 foreign and 30 local workers held hostage by Islamist terrorists at a gas facility in Algeria have reportedly managed to escape.

It is not clear if Britons were among them or how they managed to escape. AP has put the number of foreign workers who busted out of captivity at 20. Initial reports said that 41 had been taken captive by Islamic militants.

Terrorists holding dozens of foreign hostages at the plant in Algeria made some wear explosive belts and were threatening to execute those still there if Algerian forces surrounding the plant did not pull back.

According to some media reports, militants led by former al-Qaida in the Islamic Maghreb (AQIM) member Mokhtar Belmokhtar specifically threatened to kill a British national. One Briton has already been killed along with an Algerian worker.

Reports published by the Tribune de Geneve claim Algerian military snipers started targeting terrorists holding the plant, which is run by BP, Norway's Statoil and the Algerian state-owned oil company, Sonatrach.

A French hostage told France 24 by phone that the terrorists were well armed and had strapped explosives on some of the hostages.

A Briton and an Algerian were killed in the initial raid when militant group Katibat Moulathamine, or the Masked Brigade, raided the plant on Wednesday.

Algeria hostages
Terrorists attacked BP operated Ain Amenas gas field in Algeria (courtesy of BP)

Forty-one foreign nationals including Japanese, US, French, Norwegian and British workers, and about 100 Algerian workers were taken hostage at the plant, 60 miles (95km) from the Libyan border.

UK Foreign Secretary William Hague described the incident as a cold-blooded murder, and confirmed that other Britons were among the hostages.

"A number of people are held hostage there. This does include a number of British nationals and this is therefore an extremely dangerous situation," Hague said.

The natural gas complex is the third largest in Algeria.

The Masked Brigade said the attack was a retaliation for Algeria's support of France's offensive against al-Qaida-linked rebel groups in neighbouring Mali.

The group reportedly demanded freedom for 100 Islamists jailed in Algeria and asked for a safe passage to leave the country with the hostages. However Algerian authorities said they have refused the requests.

"We reject all negotiations with the group," said Algerian interior minister Daho Ould Kablia.

Algeria Launches 'Military Operation' at BP Gas Plant: Deaths Reported