Algiers, Algeria (CNN) -- Algeria launched an operation to free foreign hostages at a gas plant after it tried but failed to forge a resolution with Islamist captors, the government said Thursday.
The Algerian military ended its operation later in the day after freeing nearly 600 workers and four Algerian nationals -- two Scots, a Kenyan and a French citizen -- according to the state-run Algerian Press Service. There was no immediate indication as to how many hostages were still being held, what their condition was, or if future military operations may be forthcoming.
Algerian communications minister Mohamed Said explained earlier on state TV that the militants wanted to leave the country with the hostages as a bargaining chip, but the government refused.
Algerians and foreign workers -- including Americans and British -- were taken hostage Wednesday at a BP gas plant by Islamist militants, apparently in direct response to France's offensive in nearby Mali. The kidnappers have AK-47 rifles and put explosives-laden vests on some of the hostages, a U.S. State Department official said.
It is not clear whether the hostage takers wore the suicide vests when they staged the action, another U.S. official said.
"Situation remains very serious and difficult," said a Twitter message from the British prime minister's office.
The official Algerian Press Service (APS) report said there were numerous casualties amid the military operation, but the exact number wasn't known.
CNN could not independently confirm the report. CNN affiliate BFM-TV reported the French citizen is a nurse who worked on the site.
An Irish hostage is also free, the Irish government said, but it is not clear whether he was liberated by the Algerian military effort.
"The kidnap ordeal of Belfast man, Stephen McFaul, has ended," said the taoiseach, or Irish prime minister, Enda Kenny. "I believe he has already spoken to his family in Belfast and I wish him a safe return home to his loved ones."
An unspecified number of Americans are among the hostages held by terrorists at BP's In Amenas facility in Algeria, White House spokesman Jay Carney said.
The U.S. government is "closely monitoring the situation" and is "in contact with Algerian authorities and our international partners, as well as the BP security office in London," Carney said.
"This incident will be resolved -- we hope -- with a minimum loss of life," said U.S. Secretary of State Hillary Clinton. "But when you deal with these relentless terrorists, life is not in any way precious to them."
On Wednesday, the attackers put the number of hostages at "more than 40," including seven Americans, two French, two British and other Europeans. Another Islamist group told the Mauritanian News Agency there were 41 "Westerners."
The Algerian Press Service, though, reported that just over 20 foreign nationals were being held.
Officials from the United States, Japan and Great Britain have said some of their nationals are among the hostages.
Algerian troops fired on two SUVs trying to leave the kidnapping site, Algerian radio said, citing local sources. An Algerian reporter saw heavy clashes near the site, APS and radio reports said.
An unarmed Predator drone has flown over the BP plant to gather intelligence, a U.S. official said Thursday. Satellite imagery was taken previously.
The BP gas field is 60 kilometers (40 miles) west of the Libyan border and 1,300 kilometers from the Algerian capital, Algiers.
"The situation remains unclear and we continue to seek updates from the authorities," BP said in a statement. "Sadly, there have been some reports of casualties, but we are still lacking any confirmed or reliable information. There are also reports of hostages being released or escaping."
The company was making arrangements to get non-essential workers out of country, the statement said.
Earlier, Algeria's state media reported that all Algerian nationals who had been held hostage were free: some had fled, while others were released. The hostages still detained are foreigners, Algerian Interior Minister Dahou Ould Kablia said.
In addition to the hundreds of freed workers, 30 Algerian workers escaped -- recovered by helicopters flying over the site -- according to the GPS report.
The man behind the group claiming responsibility for the attack and kidnappings is a veteran jihadist known for seizing hostages.
Moktar Belmoktar, an Algerian who lost an eye while fighting in Afghanistan in his teens, has long been a target of French counter-terrorism forces. Libyan sources said he spent several months in Libya in 2011, exploring cooperation with local jihadist groups and securing weapons.
The militants said they carried out the operation because Algeria allowed French forces to use its air space in attacking Islamist militants in Mali. Media in the region reported that the attackers issued a statement demanding an end to "brutal aggression on our people in Mali" and cited "blatant intervention of the French crusader forces in Mali."
The fallout escalated after rebels kidnapped the Westerners, dragging governments beyond Africa into the region's conflicts and insecurity.
During the attack on the gas plant in the desert Wednesday, kidnappers killed two people -- an Algerian and a British national -- APS reported.
The rush of events sent governments scrambling to account for workers in the region.
Japan and the United Kingdom sent officials to Algeria to get the latest information. French President Francois Hollande earlier confirmed the presence of French citizens on site but would not say if any of them were hostages.
"I will not give any precision on the number of French citizens who could be held hostage. What counts now is to allow the Algerians to free them," Hollande said.
The State Department said it is still working to determine how many citizens were involved. There could be as few as three American hostages, two U.S. officials said Wednesday.
An interagency meeting on the issue was held at the White House with John Brennan, the chief counterterrorism adviser and President Barack Obama's nominee for CIA chief, a U.S. official said. The official said the Algerian government told the United States about the operation.
So far, the crisis is viewed as an "internal situation for Algeria," said the official, who emphasized Algerian security forces have successfully handled internal threats in the past.
The United States has been working through the Algerians to help resolve the crisis. There is not yet enough information to understand the situation on the ground, the official said.
"It's too early for us to do anything," he said, adding that more solid information is needed.
U.S. Defense Secretary Leon Panetta, in Europe meeting with NATO allies, called the hostage-taking "a terrorist act."
U.S. mulling requests to support French in Mali
As world powers scramble to limit the fallout, the United States is reviewing requests for support from the French in their operation in Mali, but no decisions have been made on specifics, said State Department spokeswoman Victoria Nuland.
Last week, French troops and warplanes joined Malian government forces to battle Islamist militants, who have seized most of the African nation's northern region. France, the former colonial power in Mali, said it has about 1,400 troops in what it has dubbed Operation Serval.
Europe's largest powers appeared united in their goal of removing al Qaeda-linked militants from the West African nation, where Islamist rebels are fighting to form their own territory in the north.
Nations have pledged to contribute transport planes, including Germany, Belgium and Canada. Others, such as Italy, are promising "logistical support" for the operation.
EU to help Mali's army
European Union foreign ministers Thursday agreed on a mission to train Mali's army, the EU said. It will include instructors, support staff and force protection over a 15-month period. The agency has said about 450 non-combat troops will be launched, hopefully by next month. The mission was established after Malian authorities requested help.
Hollande has said it was a "necessary decision" to go into the country.
"There are terrorist networks which, following what happened in Libya last year, have installed themselves in a large part of West Africa and are trying to destabilize the area and are involved in trafficking," he said Thursday. "Our duty is to put an end to this and France assumes its responsibility."
Hollande stressed that France was in Mali at the request of its government and within the framework of international law.
Mali was one of the most successful democracies in Africa until a coup toppled the president last year, leading the Islamists to capitalize on the chaos and establish themselves in the north.
In the quest to establish Sharia law, they have killed and mutilated residents who disobeyed them, leading the International Criminal Court to launch a war-crimes investigation.
Mali's government asked the U.N. tribunal to investigate in July, after Islamists seized much of the country, the court said.
The United Nations said it is aware of reported rights violations in Mali. It is preparing to deploy "the U.N.'s multi-disciplinary presence to Bamako, which will include a human rights component that will aim to monitor and report on alleged human rights violations." Bamako is Mali's capital.