BAMAKO, Mali -- French soldiers encircled a desert village in central Mali on Wednesday, a Malian Army colonel said, in the first direct operations involving Western troops since France began its military campaign here last week to help wrest this nation back from a militant advance.
The Malian colonel said his army's ground troops had joined the French forces and ringed the village of Diabaly, which Islamist fighters had seized the day before. Now, he said, they were engaged in trying to extricate the militants, who had taken over homes and ensconced themselves.
"It's a very specialized kind of war," said the colonel, who spoke on condition of anonymity. "The town is surrounded."
French officials have been cautious about saying exactly when the ground combat would begin. On Wednesday, a senior French defense official confirmed that a detachment of about 100 members of the French special forces were approaching Diabaly, about 250 miles north of the capital, in an effort to halt an insurgent move south and take back the town. But the official refused to confirm that an assault was yet under way.
The ground fighting expands the confrontation between the Islamists and the French forces, who have previously conducted aerial assaults after President François Hollande of France ordered an intervention in Mali last Friday to thwart a broader push by Islamist rebels controlling the north of the country.
The broadening of the military conflict came as an Algerian government official and the country's state-run news agency said that Islamist militants had seized a foreign-run gas field near the Algeria-Libya border, hundreds of miles away, taking at least 20 foreign hostages, including Americans, in retaliation for the French intervention in Mali and for Algeria's cooperation in that effort.
The Algerian agency said at least at least two people had been killed in the gas-field seizure, including one British national, and that the hostages included American, British, French, Norwegian and Japanese citizens.
Victoria Nuland, a State Department spokeswoman, told reporters in Washington, "The best information that we have at this time is that U.S. citizens are among the hostages."
Japanese officials acknowledged that Japanese citizens were involved in the hostage situation, and the Irish foreign ministry said one Irish citizen had been kidnapped. The British foreign office also said in a statement that "British nationals are caught up in this incident," which it described as "ongoing."
The twin developments underscored an earlier acknowledgment from French officials that the military campaign to turn back the Islamists and drive them from their redoubts in northern Malian desert would be a protracted and complicated one.
"The combat continues and it will be long, I imagine," the French defense minister, Jean-Yves Le Drian, said Wednesday on RTL radio. "Today the ground forces are in the process of deploying," he said. "Now the French forces are reaching the north."
Adm. Edouard Guillaud, the French chief of staff, told Europe 1 television that ground operations began overnight.
He accused jihadists of using civilians as human shields and said, "We refuse to put the population at risk. If there is doubt, we will not fire."
In Paris, Mr. Hollande said Wednesday that he took the decision to intervene last Friday because it was necessary. If he had not done so, it would have been too late. "Mali would have been entirely conquered and the terrorists would today be in a position of strength."
On Tuesday, witnesses in Mali reported, the insurgents had regrouped after French airstrikes and embedded themselves among the population of Diabaly, hiding in the mud and brick houses in the battle zone and thwarting attacks by French warplanes to dislodge them.
"They are in the town, almost everywhere in the town," said Bekaye Diarra, who owns a pharmacy in Diabaly, which remained under the control of insurgents. "They are installing themselves."
Benco Ba, a parliamentary deputy there, said residents were fearful of the conflict that had descended on them. "The jihadists are going right into people's families," he said. "They have completely occupied the town. They are dispersed. It's fear, " he said, as it became
clear that airstrikes alone will probably not be enough to root out these battle-hardened insurgents, who know well the harsh grassland and desert terrain of Mali.
Containing the rebels' southern advance toward Bamako is proving more challenging than anticipated, French military officials have acknowledged. And with the Malian Army in disarray and no outside African force yet assembled, displacing the rebels from the country altogether appears to be an elusive, long-term challenge.
The jihadists were "dug in" at Diabaly, Defense Minister Le Drian said Tuesday at a news conference. From that strategic town, they "threaten the south," he said, adding: "We face a well-armed and determined adversary."
Mr. Le Drian also acknowledged that the Malian Army had not managed to retake the town of Konna, whose seizure by the rebels a week ago provoked the French intervention. "We will continue the strikes to diminish their potential," the minister said.
Using advanced attack planes and sophisticated military helicopters, the French campaign has forced the Islamists from important northern towns like Gao and Douentza. But residents there say that while the insurgents suffered losses, many of them had simply gone into the nearby bush.
Analysts said that while forcing the insurgents from the cities was achievable, eliminating them altogether would require considerable additional effort.
"You can't launch a war of extermination against a very tenacious and mobile adversary," said Col. Michel Goya of the French Military Academy's Strategic Research Institute. "We are in a classic counterinsurrectionary situation. They are well armed, but the weapons are not sophisticated. A couple of thousand men, very mobile."
While striking the Islamists from the air, France has been steadily building up its forces on the ground: 200 more soldiers and 60 armored vehicles arrived in Mali overnight on Tuesday from Ivory Coast, bringing the total to nearly 800 soldiers. The French Defense Ministry said the force would soon number 2,500, in the vicinity of its peak Afghanistan deployment.
France is the former colonial power in Mali, and Mr. Le Drian has said it intervened to prevent the possible collapse of Mali's government and "the establishment of a terrorist state within range of Europe and of France." The French mission is aimed at supporting an African force that is still being assembled and that French officials said could begin to deploy in as soon as a week. The United States has also committed its support to the French mission.
Defense Secretary Leon E. Panetta, traveling in Spain, said that France faced a difficult task in taking on the extremists and that the Pentagon remained in talks with the French about what sort of aid was required.
The implications of the nascent French deployment -- and of the Islamist takeover of Diabaly, only about 220 miles from the capital here -- seem clear: rooting out the few thousand insurgents could well be a slog.
The Islamists are well armed, with AK-47s, rocket-propelled grenades and heavy machine guns mounted on vehicles, as well as some armored personnel carriers seized from the Malian military last year.
In the initial clashes, allied officials said, French airstrikes inflicted heavy losses on Islamist columns that could be easily identified and attacked as they advanced on roads. That led to some optimistic assessments of a rout.
But a military spokesman for the French operation in Mali said Tuesday that the Islamists had taken more territory since the French air raid began because the fighters were mixing in with the population and making it difficult to bomb without causing civilian casualties.
"It's really much too soon to tell how this fight will turn out," said an American official who has been surveying the battle from afar.
Adam Nossiter reported from Bamako, Mali; Alan Cowell from Paris; and Eric Schmitt from Washington. Reporting was contributed by Steven Erlanger and Scott Sayare from Paris, Julia Werdigier from London, and Elisabeth Bumiller from Madrid.
This article originally appeared in The New York Times.