BAMAKO, Mali — French soldiers battled Islamist occupiers of a desert village in central Mali on Wednesday, a Malian army colonel said, in the first direct ground combat involving Western troops since France began its military operation last week to help wrest Mali 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 fighting 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.”

But 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 Francois Hollande of France ordered an intervention in Mali on Friday to thwart abroader 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 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. 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, Hollande said Wednesday that he took the decision to intervene Friday because it was necessary. If he had not done so, it would have been too late.

Front Section, Pages 5 on 01/17/2013