The United States was rocked Thursday by a new crisis spurred by Islamic militants amid a deadly hostage-taking in Algeria, prompting warnings to US interests in North Africa to boost security.

Just four months after Al-Qaeda-linked extremists overran a US diplomatic outpost in Libya, the administration of President Barack Obama was anxiously watching events after militants seized hostages at an Algerian gas field.

Countries stretching from Egypt on the Red Sea, through Libya and Algeria and further south to Mali in western Africa, have been caught up in the maelstrom unleashed by the Arab Spring uprisings.

"There is a continuing effort by the terrorists, whether they call themselves one name or Al-Qaeda, to try to destroy the stability, the peace and security of the people of this region," US Secretary of State Hillary Clinton said.

Clinton, who next week will testify to lawmakers about the September attack on the Benghazi mission in Libya in which four Americans died, swiftly urged all US embassies and US firms operating in North Africa to review security.

"We are vigilant and we encourage our companies to be so as well," Lionel Johnson, vice president for Middle East and North Africa at the US Chamber of Commerce, said.

The security needs in each country were different, he stressed, as "each transition is very unique in national scope and character."

American companies have been working in the region for decades. "I think that companies that have billions of dollars of investments are not prepared to walk away from those... they are committed to being corporate citizens," he added.

US officials have repeatedly stressed that US military operations have squeezed Al-Qaeda out of safe havens in countries such as Yemen, leaving them to seek fresh strongholds in some of the world's remotest regions, often grafting themselves onto areas of existing turmoil.

"The uprising in Mali, that was going to happen," J. Peter Pham, director of the Africa Center at the Atlantic Council, said, pointing to decades of disaffection by the Tuareg population in the country.

"Libya was an accelerant if you will. And of course the secular Tuaregs who thought they were rebelling for autonomy ended up being sidelined by the extremists."

Wednesday's attack on the remote Algerian gas field near the Libyan border appeared to be retaliation for a week-old French military campaign against Islamist rebels in the north of neighboring Mali.

Algerian officials announced late Thursday that the military operation had ended, but did not provide many details on the fate of the hostages, who include an unknown number of American citizens.

Communications Minister Mohamed Said said "several people" were killed or wounded and a "large number" of hostages freed at the site, jointly operated by British oil giant BP, Norway's Statoil and state-run Algerian energy firm Sonatrach.

US officials refused to provide any details of the unfolding events in Algeria. "The situation is extremely fluid on the ground," State Department spokeswoman Victoria Nuland warned reporters.

But the audacious attack -- one of the largest hostage-takings ever in the region -- has triggered fears that Al-Qaeda in the Islamic Maghreb (AQIM), known to be sheltering in Mali, could launch even larger kidnapping operations.

"The AQIM is known for trying to kidnap Westerners. That's how they fund themselves, is through ransoming and that kind of thing," Nuland said.

According to calculations by the Atlantic Center's Pham, AQIM has pocketed about $100 million in the last decade solely from kidnapping operations.

"AQIM has been gathering resources for the better part of a decade, making literally tens of millions, if not hundreds of millions, in kidnapping ransoms, fees, protection money from narco-traffickers, and involvement in smuggling of other contra-band itself," he said.

He warned that the French operation in Mali, which has won support from Washington, would take a long time "to achieve some sort (of) equilibrium at which point you can turn it over to an African-led mission."

"France is going to be there for a long time, and it will be a long time before the Africans get their act together and deploy because of lack of resources," he said.

It would take even longer to restore a democratically elected government in Bamako, he said, calling for a proper dialogue to set the objectives of US support for the French operation.