Islamist forces on Monday launched a fresh attack in Mali's government-held south and vowed to strike "at the heart" of France to avenge a fierce military offensive against them.

French Defense Minister Jean-Yves Le Drian on Monday confirmed that Islamist rebels had seized the town of Diabali, 400 kilometers north of the capital of Mali, in the government-held south.

"We knew there would be a counter offensive towards the west," he told BFM Television. "They have taken Diabali, which is a small town, after heavy fighting and resistance from the Malian army, which was insufficiently equipped at that exact point."

"The Islamists attacked the town of Diabali today (Monday). They came from the Mauritanian border where they were bombed by the French army," said a Malian security source on condition of anonymity.

He said the Malian army had urgently dispatched a helicopter to the town.

A regional security source confirmed the attack, which he said was being led by Abou Zeid, a leader of Al Qaeda in the Islamic Maghreb (AQIM).

Le Drian said earlier Monday the Islamists had "retreated" in the east of Mali but that French forces were facing a "difficult" situation in the west where rebels are well armed.

A leader of one of the Islamist groups occupying Mali's vast north vowed revenge against France, which is on high alert and has stepped up security on home soil.

"France has attacked Islam. We will strike at the heart of France," said a leader of the Movement for Oneness and Jihad in West Africa (MUJAO), an offshoot of AQIM.

Asked where they would launch attack, Abou Dardar told AFP by telephone that "Everywhere. In Bamako, in Africa and in Europe."

Another MUJAO leader Omar Ould Hamaha, nicknamed "Redbeard", warned on radio Europe 1 that France had "opened the doors of hell" with its intervention and faced a situation "worse than Iraq, Afghanistan or Somalia."

In Bamako the French high school was closed on Monday as a "precautionary measure," French ambassador Christian Royer said.

France's intervention has been backed by the EU, NATO and the US, while Britain is providing logistical support in the form of transport planes.

Germany said Monday it was considering ways to help France in its mission in Mali such as providing logistical, medical or humanitarian aid.

NATO said Monday it supported French efforts to turn back the terrorist threat in Mali but that the alliance had received no request for assistance. The EU ruled out taking any combat role on Monday.

EU leaders in December approved a plan to send some 250 military trainers for the Malian army. "The training mission still makes sense. It is even more urgent to launch our mission," said EU foreign policy spokesman Michael Mann.

AFP - Reuters