Islamist fighters in Mali captured a central town on Monday as a ferocious campaign of air strikes by French warplanes aimed at halting their advance entered its fourth day.

Despite intensive bombardments, the fundamentalist insurgents pushing south towards the capital, Bamako, overran the central town of Diabaly, just 250 miles to the north.

An Islamist militant leader warned the French government its intervention in Mali had opened the "gates of hell".

France's defence minister, Jean-Yves Le Drian, said that in a counter-offensive heavily armed rebels "took Diabaly after fierce fighting and resistance from the Malian army that couldn't hold them back".

France immediately extended its bombardment of the Islamists with air strikes in central Mali.

While officials in Paris declared they were "satisfied" with Operation Serval, as the French military intervention is codenamed, the military was also reporting unexpectedly fierce resistance in the west of the country.

Le Drian said the situation was "evolving favourably", but admitted: "There remains a difficult point to the west where we have to deal with extremely well-armed groups."

The Islamists' advance on Monday came as fighter jets dropped bombs and strafed their camps and convoys.

"We know that the nub, the most important [action] will happen towards the west," Le Drian added. "We bombed towards the west overnight and we are continuing bombing towards the west today because it's here that the most important fighting is taking place.

"The forces of the terrorist groups are exactly what we expected. They are heavily armed. They are very determined. They are very well organised. We knew this," he said.

The French radio station Europe 1 broadcast a telephone interview with Omar Ould Hamaha, an Islamist militant leader. He said the French government had opened the "gates of hell" and "fallen into a trap much more dangerous than Iraq, Afghanistan and Somalia". Hamaha, known as "Red Beard" because of his hennaed hair, added: "And it's only just started."

Hamaha goaded French fighters: "Come down to earth if you are men," he said. "We'll welcome them with open arms."

In Bamako, where a state of emergency remains in place, there was a heavy military presence on the streets. Civilian trucks, vans and transport vehicles have been commandeered by the army in recent days to ferry personnel around the city.

Soldiers from other African nations have been seen in Bamako, and were due to be sent north to support the Malian army, but residents in the key strategic town of Mopti said they had yet to arrive.

"We are hearing reports that troops from Burkina Faso are driving by land to Mopti and will be arriving in the next few hours," said Issa Ballo, a Mopti resident.

"Things have quietened in Mopti since the French arrived, but the roads around all the military camps are blocked and the area around the airport is sealed off."

France has been carrying out air attacks on the Islamists in the north of Mali since Friday evening in an attempt to stop the militants gaining control of the country. Paris justified the intervention on the grounds of maintaining stability in the region and reducing the risk of terrorist attacks elsewhere, including France.

About 500 French ground troops are already in Bamako and others are expected to follow. Reinforcements are expected from neighbouring African countries including Nigeria, Niger, Burkina Faso, Togo, Senegal and Benin.

The EU will hold an emergency meeting on Thursday to assess the situation, the EU foreign affairs chief, Catherine Ashton, said. "I have convened an extraordinary Foreign Affairs Council this week to take stock of possible EU actions in support of Mali (government) ... to help it cope with the current situation," she said in a statement.

Earlier, an EU spokesman, Michael Mann, said the EU was speeding up its preparations for a troop training mission but was not planning any combat role.

The US defence secretary, Leon Panetta, said the US was providing intelligence-gathering assistance to the French in Mali.

The north of Mali fell under the control of Islamists nine months ago. As the international community dithered over what to do, France decided to launch a campaign of air strikes after rebel convoys were reported to be moving south towards Bamako.The French strikes are being carried out by Mirage 2000 fighter jets based in Chad and Gazelle helicopter gunships. Rafale jets are also reported to be flying bombing missions from France.

A group of independence-seeking Touaregs has said it will support the Malian government and French in the battle against the Islamists. The MNLA (Mouvement National pour la Libération de l'Azawad) said it was "ready to help" the French in their "attempts to end terrorism in the Azawad". The Azawad is the northern region occupied by the Islamists.

"We totally support the French air campaign. Of course we are ready to help the French army work on the ground," one of the MNLA's leaders said on Monday.

"Because of our knowledge of the ground and the populations, we are more useful a force than that of CEDEAO (Communauté Economique des Etats de l'Afrique de l'Ouest), which will be deployed to back up the Malian army," he added.

On Monday, the French military bombed Islamist bases in Douentza, 500 miles north of Bamako, for the fourth day running. However, the fundamentalists were reported to have already fled the town.

The air campaign was said to have had more success at the northern Islamist camp at Gao. "At the Gao military camp there have been deaths. The Islamists were taken by surprise in the middle of a meeting. There were a lot killed," a Malian regional security source told AFP. "They lost an enormous amount of logistics and men. The figure of 60 deaths isn't an exaggeration at Gao and might even be higher."

Patrick Smith, editor of Africa Confidential, the authoritative newsletter covering African economic and political affairs, who was in Mali in November, told the Guardian the French military operation raised more questions than it answered. "I'm not surprised about the military intervention. There were French special forces on the ground for the best part of the last year. They were there when I was in Mali and had spotted a couple of French passport holders heading for the north to join the jihadists," he said.

"It was clear they had been tracking them. It wasn't a surprise either to all the aficionados who think France has decided to ramp this up into a full-scale emergency, zap the hell out of the jihadists and shut them up for a while to give the African forces time to get organised. The big danger is thinking bashing these [Islamist] camps to bits means the problem is over.

"There is a genuine fear that these people could come from north Mali and set off bombs on the Champs Elysées."

Smith said it was hard to quantify support for the Islamic fundamentalists among the Malian population.

"It's unknowable. Some people will tell you they have no support at all and it's all at the point of a gun, others that they do have support.

"What they do have is a lot of money. Tens of millions of dollars from people and cocaine smuggling, so they can pay much better wages than the state army. They may not have the hearts and minds, but they certainly have the dosh."The humanitarian organisation Médecins Sans Frontières said on Monday it was treating civilians injured in the conflict and was worried about those living close to the combat zones.

"We call on all parties to the conflict to respect the safety of civilians and to leave medical facilities untouched," said Rosa Crestani, MSF's emergency response co-ordinator.Save the Children warned that women and children were being forced to flee for their lives were among the poorest and most vulnerable in the country. Families forced from their homes are adding to nearly 350,000 people who fled the region after last year's fighting erupted.