But the real number of U.S. hostages could be as few as three, two U.S. officials said Wednesday.
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."
Defense Secretary Leon Panetta, in Europe meeting with NATO allies, called the incident "a terrorist attack."
Nine or 10 Americans were working at the site, 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 being held prisoner be released and sent to northern Mali. The official was not clear about where the prisoners were.
"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."
He said 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.
A U.S. official said the capability existed "to see the area" around the facility and that the ability to monitor the situation will "improve soon."
The CIF unit is prepared to move within four hours of being ordered to do so, a U.S. defense official told CNN. The official said that, before the hostage incident, the unit had been headed to Senegal, where it was to be on standby for missions in Mali, but that it may now move elsewhere.
A second U.S. official said that any operation would take time to unfold. "This is not the type of mission that you plan and execute overnight, it's just not," the official said. "We don't know specifically how many militants or hostages there are. But with so little information, if you're talking dozens of militants and up to 40 hostages, I don't see how you go in without killing half the hostages."
The second official said that Marine Corps' Fleet Antiterrorism Security Teams would be deployed only to augment security at any nearby embassies. None is under threat. They would not be deployed for field operations in any rescue attempt, the official said.
The Pentagon's Africa Command is taking the military lead on the matter, and the State Department and FBI are the lead agencies in contact with the Algerian government, a Defense official said.
The official said commanders have assets "not too far away," including aircraft from the Air Force's base in Aviano, Italy, and a Marine Corps FAST team in Sigonella, Italy.
Any action would need to be approved by Algeria, "which is more than just a formality," the official said.
A spokesman for the group -- whose name means "those who sign with blood" -- said that 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 Minister's 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.