Islamist militants holding dozens of Western hostages at a gas plant in Algeria have forced some to wear belts strapped with explosives, French television said on Thursday, quoting one of the hostages.

France 24 said the man also told the channel during a telephone call late on Wednesday that the hostage-takers were heavily armed and had threatened to blow up the natural gas facility if the Algerian army tried to free the hostages.

"They attacked the two sites at the same time. They went inside and once it was daylight they gathered everybody together," the man, who sounded calm, said in the only part of the phone call that France 24 aired.

Gunmen stormed the gas pumping site and workers' housing before dawn on Wednesday and a group calling itself the "Battalion of Blood" has claimed it is holding 41 foreigners including Americans, Japanese and Europeans at Tigantourine, deep in the Sahara.

The French government has not confirmed whether there are French nationals among the hostages.

Europe 1 radio said that according to its sources four French nationals could be among the hostages.

France 24 said it had no way of verifying whether the man it spoke to, an employee at the site who declined to be named, was speaking under duress.