Fighting between the Malian army and Tuareg rebels in Mali's northern region of Kidal left at least 36 people dead, eight of them soldiers, while 87 others were injured, Malian Defense Minister Soumeylou Boubeye Maiga confirmed on Sunday.

The fighting broke out before Malian Prime Minister Moussa Mara's visit to Kidal early on Saturday and he was forced to take shelter in an army base.

In a statement on Sunday, the minister said forces securing the prime minister's visit were attacked by rebel MNLA fighters supported by terrorist elements.

Mali sent in troops on Sunday to retake Kidal from Tuareg separatists, according to the United Nation.

"In light of this declaration of war, the Republic of Mali is henceforth at war," Mara said.

He told a news conference on Sunday that the government had already sent troops, including special forces, to retake Kidal.

"Reinforcements are on the way to Kidal. The objective is to totally retake Kidal," a senior military source said, asking not to be named.

Mara was visiting the town, a stronghold of Tuareg separatists, for the first time since his appointment last month as part of efforts to revive long-delayed talks with northern armed groups.

Mali was thrown into turmoil in 2012 when al-Qaida-linked Islamists took advantage of a Tuareg-led rebellion and seized control of the country's north before a French-led military operation drove them back last year.

The government and a grouping of armed groups including the MNLA, which broke with the Islamists ahead of the French offensive, signed an agreement to hold talks over autonomy last year.

But the clashes now threaten to sink efforts to find a peaceful solution to the long cycle of rebellions in the West African nation's desert north.

The United States condemned the violence, saying it undermined the country's fragile peace.

"We call for the immediate release of all hostages, and urge all parties to refrain from violence and from any acts that place civilians at risk," US State Department spokeswoman Jen Psaki said. "The way to resolve these issues is through an inclusive and credible negotiation process, not through violence and intimidation."

The flare-up in a trouble spot many had hoped had been brought under control comes as West African nations and their international partners are redoubling efforts elsewhere to contain Islamist groups such as Boko Haram in Nigeria.

France, in particular, is seeking to redeploy part of its force in Mali to tackle the regional threat.

MINUSMA, a nearly 13,000-strong UN peacekeeping mission, is rolling out but is not yet at full strength.

Mara criticized the French and UN forces for allowing the attack to take place.

"The very least we'd expected ... was that they'd ensure the governor's office wasn't attacked," he said.

Xinhua-Reuters