You are not permitted to download, save or email this image. Visit image gallery to purchase the image.
The raiders were also reported to have killed three people, including a Briton and a French national.
An al Qaeda affiliated group said the raid had been carried out because of Algeria's decision to allow France to use its air space for attacks against Islamists in Mali, where French forces have been in action against al Qaeda-linked militants since last week.
The attack in southern Algeria also raised fears that the French action in Mali could prompt further Islamist revenge attacks on Western targets in Africa, where al Qaeda in the Islamic Maghreb (AQIM) operates across borders in the Sahara desert, and in Europe.
AQIM said it had carried out Wednesday's raid on the In Amenas gas facility in OPEC member Algeria, Mauritania's ANI news agency reported.
The Algerian interior ministry said: "A terrorist group, heavily armed and using three vehicles, launched an attack this Wednesday at 5 a.m. against a Sonatrach base in Tigantourine, near In Amenas, about 100 km (60 miles) from the Algerian and Libyan border."
"The Algerian authorities will not respond to the demands of the terrorists and will not negotiate," Interior Minister Daho Ould Kablia was quoted as saying by official news agency APS.
The gas field is operated by a joint venture including BP , Norwegian oil firm Statoil and Algerian state company Sonatrach.
BP said armed men were still occupying facilities at the gas field, which produces 9 billion cubic metres of gas a year(160,000 barrels of oil equivalent per day), more than a tenth of the country's overall gas output, and 60,000 barrels a day of condensate.
"The site was attacked and occupied by a group of unidentified armed people at about 0500 UK time. Contact with the site is extremely difficult, but we understand that armed individuals are still occupying the In Amenas operations site," it said.
APS said a Briton and an Algerian security guard had been killed and seven people were injured. A French national was also killed in the attack, a local source said.
Also among those reported kidnapped by various sources were five Japanese nationals working for the Japanese engineering firm JGC Corp, a French national, an Irishman, and a number of Britons.
The U.S. State Department said it believed some U.S. citizens were also among the hostages, while Norway said 13 of its nationals were involved.
A member of an Islamist group styling itself the "Blood Battalion" was quoted by Mauritanian media as saying that five of the hostages were being held at the gas facility and 36 were in a housing area. APS said the Islamist raiders had freed Algerians working at the gas facility.
"The operation was in response to the blatant interference by Algeria and the opening of its air space to French aircraft to bomb northern Mali," the Islamist spokesman told Mauritania's ANI news agency.
ANI, which has regular direct contact with Islamists, said that fighters under the command of Mokhtar Belmokhtar were holding the foreigners.
Belmokhtar for years commanded al Qaeda fighters in the Sahara before setting up his own armed Islamist group late last year after an apparent fallout with other militant leaders.
The Algerian army was in the area of the gas facility, according to French and Algerian sources.
ANI also reported that the Islamists said they were surrounded by Algerian forces and warned that any attempt to free the hostages would lead to a "tragic end". One of the hostage takers told ANI that the perimeter of the site had been mined.
The attack was the first time in years that Islamist militants are known to have launched an attack on an Algerian energy facility.
The attack could have implications for security across the whole of Algeria's energy sector, which supplies about a quarter of Europe's natural gas imports and exports millions of barrels of crude oil each year.
Such an attack would require a large and heavily armed insurgent force with a degree of freedom to move around, all elements that al Qaeda has not previously had.
However, the conflict in neighbouring Libya in 2011 changed the balance of force. Security experts say al Qaeda was able to obtain arms, including heavy weapons, from the looted arsenals of former leader Muammar Gaddafi.
Statoil, a minority shareholder in the gas venture, said it had 17 employees at the plant and four of them had been evacuated. The company declined to comment on the other 13.
The five Japanese work for the engineering firm JGC Corporation, Jiji news agency reported, quoting company officials. JGC has a deal with Sonatrach-BP-Statoil Association for work in gas production at In Amenas.
A reporter for Japan's NHK television managed to call a JGC worker in Algeria.
The worker said he got a phone call from a colleague at the gas field. "It was around 6 a.m. this morning. He said that he had been hearing gunshots for about 20 minutes. I wasn't able to get through to him since."
French troops launched their first ground operation against Islamist rebels in Mali on Wednesday in an action to dislodge from a strategic town al Qaeda-linked fighters who have resisted six days of air strikes.