Oil giant BP, which operates the In Amenas field in a joint venture with Sonatrac, the Algerian national oil company, and Norway's Statoil, said it was attacked by "unidentified armed people" who were occupying the site.
Statoil Executive Vice President Lars Christian Bacher said 17 of its employees -- 13 of them Norwegian -- were in or around the facility at the time of the attack. "We have received information that five of the 17 are brought to safety in a military camp in the area," he said in a statement.
Four are Norwegian and one is a Canadian resident; two people were injured and have received medical treatment, he said.
The gas field lies about 60 kilometers west of the Libyan border and some 1,300 kilometers from the capital, Algiers, BP said.
The attack comes four days after Libyan, Algerian and Tunisian prime ministers reached security agreements during a summit in Libya, where they agreed to work together against terrorist threats.
In a statement, Rep. Ed Royce, R-California, called the attack "the latest demonstration of a large and growing radical movement across North Africa" and said the French "have acted appropriately" in intervening in Mali.
"Al-Qaeda in the Islamic Maghreb and its offshoots have been conducting regular kidnappings for years -- financing much of its operations through ransoms, earning millions," the chairman of the House Foreign Affairs Committee said.
"The network has planted deep roots in Europe. In recent years, cells have been broken up by authorities in France, Germany, Italy, Spain and Switzerland. In Mali, its brethren are imposing the strictest interpretation of Islamic law -- banning music and chopping off limbs. Large amounts of weapons are flowing into the region."