Malian Tuareg soldiers loyal to El-Hadj Gamou listen during a visit by Mali's army chief of staff in Kidal. File picture: Rebecca Blackwell

Bamako - Mali has sent 1 500 troops to retake the rebel stronghold of Kidal, military sources said Monday, after Tuareg separatists laid siege to local government offices, sparking a gun battle in which dozens of people were killed.

Eight soldiers and 28 insurgents died in fighting Saturday outside the regional governor's offices while around 30 civil servants were being held hostage by the militants, the government said.

“Fifteen hundred Malian soldiers arrived in Kidal in the last 24 hours. They are continuing to come,” a foreign military source told AFP.

“They have come with weapons and luggage, many people and many weapons,” the source added.

A defence ministry official confirmed the information, saying the figure was set to rise.

“Our soldiers will defend the country, by force, if necessary,” he added.

Prime Minister Moussa Mara, who was in Kidal over the weekend as part of a first visit to the restive north since his appointment, said on Sunday that terrorists had “declared war on Mali”.

“We will mobilise the resources to fight this war,” Mara told AFP by telephone.

Former colonial power France demanded the “immediate and unconditional release” of the hostages, as did the United States, while Albert Koenders, the head of the United Nations' MINUSMA peacekeeping force in Mali, condemned the violence.

“We are staying in our home to be on the safe side. We don't know what's going to happen and we are frightened,” a resident of Kidal told AFP.

The Malian government has blamed the clashes on Tuareg separatists but Mara said Islamist militants had taken advantage of the crisis “to participate in the chaos alongside other terrorist groups”.

He said the government was working to get the hostages released but added that some had been “killed in cold blood” while others were freed as they had been wounded.

Mara said the governor's offices had been attacked by “jihadists, terrorists... with the clear aim to destroy and kill”.

Malian troops “reacted accordingly. Today Malian armed forces are in Kidal, (they) are readying themselves for any contingencies,” he said.

Mara was due to meet Islamic clerics on Monday, an aide said, to discuss “future field operations”.

The premier was keen to underline that Mali was prosecuting a war against terrorists, not Muslims, the source said.

Kidal, 1 500 kilometres northeast of the capital Bamako, was the scene of anti-government protests by several hundred people on Friday and Saturday.

Sporadic gunfire was heard overnight but calm had returned by morning, a local government official told AFP.

President Ibrahim Boubacar Keita is expected to raise the crisis in a televised address to the nation Monday.

Malian Defence Minister Soumeylou Boubeye Maiga identified the rebels on Sunday as members of the Tuareg separatist National Movement for the Liberation of Azawad (MNLA), who he said were “supported by members of terrorist groups”.

“Our forces have taken control of all government buildings except, for the moment, the governor's offices,” he said.

The MNLA said it was holding hostage the regional director of Kidal, a prefect, the governor's adviser and 24 soldiers, promising “humane treatment” to its captives.

Following a June peace deal that paved the way for presidential elections, the separatists evacuated the governor's offices in November last year after a nine-month occupation.

But the process deeply divided the MNLA, whose ultimate goal is the independence of Azawad, the minority Tuareg name for their homeland in northern Mali.

Up until the agreement, the Tuareg group had refused to allow any government soldiers or civil servants into the desert town.

The country descended into crisis in January 2012, when the MNLA launched the latest in a string of Tuareg insurgencies in the north.

A subsequent coup in Bamako led to chaos, and militants linked to Al-Qaeda overpowered the Tuareg to seize control of Mali's northern half.

A French-led military operation launched in January 2013 ousted the extremists, but sporadic attacks have continued, and the Tuareg demand for autonomy has not been resolved.

Foreign troops in Mali have come in for some criticism since Saturday, with Twitter users wondering under the #MINUSMerde hashtag why peacekeepers had seemed unable to repel the rebels.

“We demand that you leave Kidal. Your agenda is not that of Mali. We refuse the de facto partition of the country,” a tweeter identifying himself as an architect named Harouna Traore said.

At a protest in Gao, Mali's largest northern town, demonstrators in their hundreds chanted “down with MINUSMA” and “Free Kidal”, witnesses told AFP.