Islamist insurgents have advanced towards the Malian capital, Bamako, despite intense bombardment by French warplanes, as ministers in Paris admitted their forces were meeting more resistance than expected.

Fighting continued on Monday morning as French aircraft and Malian government troops sought to repel rebel forces who overran the garrison town of Diabaly, in central Mali, according to the French defence minister, Jean-Yves Le Drian.

"They took Diabaly … after fierce fighting and resistance from the Malian army that was not able to hold them off at that moment," Le Drian told BFM television.

Earlier he said the military operation had encountered fierce opposition in the west of Mali, where there were "extremely well armed groups".

Le Drian said: "We knew that the key spots would be towards the west and it's in the west where we were bombing last night, and it's in the west today where the most important fighting is going on."

The rebels had been bottled up in the narrow neck of central Mali, about 420 miles from Bamako, but by sweeping in from the west they are now 250 miles from the capital.

Mali map

Nato welcomed the French intervention but said it had received no request for help from Paris. "There has been no request, no discussion [within Nato] on the situation in Mali; the alliance as such is not involved in this crisis," said a spokesperson, Oana Lungescu.

About 500 French ground troops are already in Bamako and others are expected to follow. The official line is that they are there to secure the airport and to carry out any necessary evacuation of the 6,000 French people living there.

But the Elysée has acknowledged that part of this ground force will head towards the centre of the country to block any attempt by the insurgents to break through. It is also reported that French troops may move into the contested north to fight alongside the Malian army. This last option was approved by François Hollande at a special defence council meeting on Monday morning.

Reinforcements are expected from neighbouring African countries including Nigeria, Niger, Burkina Faso, Togo, Senegal and Benin. Even Algeria, at first reticent, has agreed to allow French planes to use its airspace, although the Algerian press has been fiercely critical of Paris's intervention.

"The French military operation in Mali has been given the code name Serval. The serval is a small African cat whose particularity is to urinate 30 times an hour to mark its territory," the Liberté newspaper said. It accused Paris of doing the same.

Islamist forces coming under heavy bombardment by French planes in the north of Mali have threatened retaliation to "hit the heart of France". Omar Ould Hamaha, a spokesman for the Movement of Unity and Jihad in West Africa, one of the main factions in the rebel alliance, said on Europe 1 radio: "France has opened the gates of hell for all the French. She has fallen into a trap which is much more dangerous than Iraq, Afghanistan or Somalia."

The French interior minister, Manuel Valls, said the threats had prompted him to raise the national security rating, to bright red, one grade down from the highest level, scarlet.

"Since the bombings in London in 2005 it has been at a high level, level red. During such a long period, our attention can lapse, so we have decided to raise the vigilance in public places and on public transport by increasing security patrols," Valls told Le Parisien. "We won't diminish the threat by giving in to terrorists. Like other countries, France is regularly targeted by groups calling for global jihad."

Mathieu Guidère, a specialist in strategic intelligence at the University of Geneva and author of the book The New Terrorists, said he believed the Malian Islamists had been arming and training since August for a French intervention.

He said they had obtained weapons from three sources. From Libya after the fall of Muammar Gaddafi they had obtained mostly light weapons including Kalashnikovs, machine guns, rockets and surface-to-air missiles, along with jeeps and pickup trucks. "These were primarily used in the first battles, where they captured Timbuktu, Gao and Kidal against the Malian army. But there's not much left," he told Le Nouvel Observateur.

The second consignment of arms was captured when the Malian army fled Gao as the Islamist forces captured the city. "The soldiers abandoned their bases with their weapons. The Islamists quite simply gathered up the armaments of the regular army: a few tanks, artillery, batteries. These weapons haven't yet been used," Guidère said.

The third wave of weapons was more recent, he said. "Since August, when France appeared more active and on the offensive on the subject [of Mali], the Islamists have been preparing for the possibility of an intervention. They began to use money collected from various trafficking of drugs and hostages. With this small capital they have, since September, bought everything they could on the black market. They approached all the dealers in the region, notably the Nigerians, and those from Chad and Libya."

He said there were also a lot of Russian arms in the region. "The Russians on learning of a western military operation let their arms dealers in Africa sell anything and everything. Here, the Islamists have a stock of modern material, [effective] and efficient, especially against helicopters and tanks, including night-vision glasses and binoculars."

Asked how France could have been unaware of the enemy's capacity, Guidère said: "It's true that the last important [weapons] acquisitions were on the black market and recent, but expressing astonishment is also part of the ministry of defence's communications operation. How else to justify the loss of a soldier so quickly, and the knocking out of a Mirage 2000 fighter jet against people with only Kalashnikovs."

• This article was amended on Tuesday 15 January 2013. A second consignment of arms was captured when the Malian army fled Gao, not Barnako. This has been corrected.