Declaring France had “opened the gates of hell” with its assault, the rebels threatened retribution.

“France . . . has fallen into a trap much more dangerous than Iraq, Afghanistan or Somalia,” said Omar Ould Hamaha, a leader of the Movement for Oneness and Jihad in West Africa, one of the rebel groups controlling the north, speaking on radio Europe 1.

French fighter jets have been pummeling the insurgents’ desert stronghold in the north since Friday, determined to shatter the Islamist domination of a region many fear could become a launch pad for terrorist attacks on the West and a base for coordination with al-Qaida in Yemen, Somalia and Pakistan.

The Islamist fighters responded with a counter-offensive Monday, overrunning the garrison town of Diabaly, about 100 miles north of Segou, the administrative capital of central Mali, said French Defense Minister Jean-Yves Le Drian.

The French Embassy in Bamako immediately ordered the evacuation of the roughly 60 French nationals in the Segou region, said a French citizen who insisted on anonymity out of fear for her safety.

France expanded its aerial bombing campaign, launching airstrikes for the first time in central Mali to combat the new threat. But the intense assault, including raids by gunship helicopters and Mirage fighter jets, failed to halt the advance of the rebels, who were only 250 miles (400 kilometers) from the capital, Bamako, in the far south.

The rebels “took Diabaly after fierce fighting and resistance from the Malian army, that couldn’t hold them back,” said Le Drian, the French defense minister.

Mali’s military is in disarray and has let many towns fall with barely a shot fired since the insurgency in the West African nation began almost a year ago. While the al-Qaida-linked extremists control the north, they had been blocked in the narrow central part of the landlocked nation.

They appear to have now done a flanking move, opening a second front in the broad southern section of the country, knifing in from the west on government forces.

In response to the insurgent advances, Mauritania, which lies to the northwest of Mali, put its military on high alert. To the south, the nation of Burkina Faso sent military reinforcements to its border and set up roadblocks. Even Algeria, which had earlier argued against a military intervention, was helping France by opening its air space to French Rafale jets.

Many of Mali’s neighbors, who had been pushing for a military intervention to flush out the jihadists, had argued that airstrikes by sophisticated Western aircraft would be no match for the mixture of rebel groups occupying northern Mali.

Leaders of ECOWAS, the regional body representing the 15 nations in western Africa, stressed that the north of Mali is mostly desert, and that it would be easy to pick off the convoys of rebel vehicles from the air since there is almost no ground cover.

Monday’s surprise assault and the downing of a French combat helicopter by rebel fire last week have given many pause. Just hours before Diabaly fell, a commander at the military post in Niono, the town immediately to the south, laughed on the phone, and confidently asserted that the Islamists would never take it.

By afternoon, the commander, who could not be named because he was not authorized to speak publicly, sounded almost desperate. “We feel truly threatened,” he said.

He said the rebels approached Diabaly from the east, infiltrating the rice-growing region of Alatona, which until recently was the site of a large, U.S.-funded Millennium Challenge Corporation project.

French aircraft bombed a rebel convoy 25 miles from Diabaly late Sunday, the commander said. “This morning we woke up and realized that the enemy was still there. They cut off the road to Diabaly. We are truly surprised – astonished,” he said.

It was unclear what happened to the Malian troops based at the military camp in Diabaly. The commander said that he had not been able to reach any of the officers at the base, raising fears they were massacred.

A French squadron of about 150 troops and armored vehicles stationed in neighboring Ivory Coast was headed to Bamako to help with the offensive in Segou, said Col. Thierry Burkhard, a spokesman for the French military in Paris. The troops were joining the 550 French forces already in Mali, said an African diplomat, speaking on condition of anonymity because he was not authorized to speak publicly.

The French national being evacuated from Segou said the email she received from the French Embassy indicated that small groups of rebel fighters were already heading to Segou, a drive that normally takes two to three hours.

Mali’s north, an area the size of France, was occupied by al-Qaida-linked rebels last April following a coup in the capital. The international community has debated what to do, with most foreign powers backing a U.N. Security Council resolution in December that called for training the Malian armed forces before any military intervention was launched. Diplomats said no intervention could happen before September.