(CNN) -- Islamists angry over Algeria's support for the French offensive in Mali attacked a gas field in southern Algeria, killing two people and seizing a number of hostages, including Westerners, Algeria's interior minister told Algerian state television Wednesday.
Diho Weld Qabliyeh said Westerners accompanied by a group of Algerian security forces were en route to Ain Menas Airport when another group of no more than 20 people began shooting at their convoy. The security forces returned fire and the attackers withdrew to the base of the petroleum operation, some three kilometers away.
Upon arrival at the base, they attacked it "and took in a number of Westerners and Algerians -- some people told us they were nine, some people told us 12."
But accounts differed over the number.
An Islamist group claiming responsibility for the attack told the Mauritanian News Agency and Sahara Media that 41 "Westerners including seven Americans, (as well as) French, British and Japanese citizens have been taken hostage."
But the Algerian Press Service, citing a source from the provincial administration of Illizi, reported that "a little more than 20 foreign nationals are held hostage."
An Algerian and a Westerner were killed in the attack and two Westerners, two of the security forces and two guards from the base were wounded, he said.
Algerian media reported that a British national was among the fatalities.
Algerian military forces were surrounding the building holding the hostage takers and the hostages, Qabliyeh said.
Late Wednesday, the hostage-takers released Algerians being held, but continued holding the Westerners, Algerian state television reported.
The group has issued demands, Qabliyeh said. "The authorities do not negotiate, no negotiations, we have received their demands, but we didn't respond to them," he added.
U.S. State Department spokeswoman Victoria Nuland confirmed the incident and said Americans were among the hostages, but gave no further details.
She said the United States was in touch with Algerian authorities and monitoring the situation. The U.S. Embassy issued an emergency message to U.S. citizens in that country to be careful, she said.
Some nine or 10 Americans were working at the field and U.S. officials were trying to determine who had been abducted, a State Department official told CNN.
The source said the abductors were demanding that members of their group who are being held prisoner be released and sent to northern Mali. The official was not clear about where the prisoners were being held.
"The first priority is to gain understanding of what is happening," a senior U.S. official with direct knowledge of the matter said. "We are working on ways to improve that now."
The official added that a Special Operations team -- the Commanders In extremis Force -- "is on a very short string." In addition, he said, "other nations are similarly assessing their response posture."
He continued that "another important piece will be for Libya to really lock-down the nearby border on their side," so personnel and weapons cannot get through.
Defense Secretary Leon Panetta, in Europe meeting with NATO allies, called the hostage-taking "a terrorist attack."
A spokesman for the group -- the name of which was translated as "those who sign with blood" -- said that the jihadists controlled the plant.
Saying that the operation was an act of revenge against Algeria, the spokesman said 400 Algerian soldiers were on the site "who have not been targeted by jihadists."
A spokesman for the Norwegian Prime Ministers' office told CNN that "13 Norwegians, all employees of Statoil, are involved in the incident at Amenas gas field in Algeria."
The UK Foreign Office said British nationals were caught up in the incident.
Ireland's foreign minister said there were reports that an Irish citizen was involved; the office of the French president refused comment on reports that French citizens were among the hostages.
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.
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 Saturday in a summit in Libya, where they agreed to work together against terrorist threats.