Armed insurgents exchanged fire with soldiers in Mali, plunging peace negotiations into disarray in the first clash this year between government forces and the main Tuareg rebel alliance.

A Malian army colonel said fighters from the Tuareg-led Coordination for the Movements of Azawad (CMA) had ambushed military positions in Lere, a central town near the Mauritanian border.

"Groups of armed Tuareg rebels have been attacking us since 4:00 pm (1600 GMT). We are fighting back and defending our positions," a Malian army colonel told AFP from the town.

The CMA had warned in a statement after it came under attack from a pro-government militia on Tuesday it had "no other choice than to use its right to exercise legitimate self-defence to protect civilians, its people and its positions".

The army source said the militants had arrived in several vehicles from the west of the town.

"We are under fire. Everybody is hiding at home. The rebels are shooting, the Malian army too," a local councillor told AFP by telephone as gunshots rang out in the background.

The attacks came on a day of violence which started when unidentified militants opened fire on troops at a national guard camp in northern Mali, killing two soldiers and a child, security sources told AFP.

The gunmen struck at 5:00 am in the former Islamist stronghold of Goundam, 80 kilometres (50 miles) from the ancient desert city of Timbuktu, a Malian security source said.

The head of MINUSMA, the United Nations peacekeeping mission in Mali, had voiced concerns for the west African nation's fragile peace process ahead of the attack.

UN Secretary-General Ban Ki-moon echoed those concerns Wednesday, calling for an "immediate cessation of the hostilities in northern Mali" following "serious ceasefire violations" in recent days, according to a statement.

- Deeply divided -

On Monday, the pro-government Imghad and Allies Tuareg Self-Defence Group (GATIA) was accused of violating a ceasefire when it seized key rebel positions in the desert town of Menaka, while UN vehicles were attacked by rebels on Tuesday.

The situation was calm on Wednesday in Menaka, where GATIA fighters have occupied the town hall, a MINUSMA source said.

Separate MINUSMA sources said militants appeared to have driven from the east ahead of Wednesday's attack in Goundam, adding that they stole cars belonging to the army and the government.

Mali was upended by a coup in 2012 which opened the door for Tuareg separatists to seize the towns and cities of the vast northern desert with the help of several Islamist groups.

The Tuareg were then sidelined by their one-time allies, extremists who imposed a brutal version of Islamic shariah in the region and destroyed historic buildings and artifacts in Timbuktu.

Ansar Dine (Defenders of Faith) took control of Goundam and destroyed shrines in the town of around 20,000, declaring them idolatrous.

The Islamists then pushed south toward Bamako, prompting France to deploy troops in January 2013 who drove them back into the country's mountains and vast desert, and Mali returned to democracy with the election in August of President Ibrahim Boubacar Keita.

The country remains deeply divided, however, with the impoverished north home predominantly to lighter-skinned Tuareg and Arab populations who accuse the sub-Saharan ethnic groups that live in the more prosperous south of marginalising them.

- Threat to peace -

The recent violence comes amid assurances by the CMA that it is committed to a deal to bring stability to Mali.

The government and a coalition of armed groups from the north known as the Platform have already signed the peace accord, brokered by Algeria under UN auspices over the past eight months.

But the CMA has been holding out ahead of the May 15 deadline, demanding an amendment guaranteeing political recognition for "Azawad", the name used by the Tuareg for the northern part of Mali.

"Unless there is a last-minute change, the CMA will initial the agreement before May 15," a Western diplomat in Bamako told AFP.

"In the interim, the CMA and government will hold direct talks."

The diplomat said 15 heads-of-state had been invited to the finalising of the accord in Bamako on May 15.

MINUSMA chief Mongi Hamdi said on Tuesday he had met rebel leaders over the weekend in the Mauritanian capital Nouakchott who "confirmed their intention to initial the agreement".

But he voiced concerns that "months of intense negotiations involving all parties to put an end to the Malian crisis could be threatened" by renewed violence.

Mali's various rival factions agreed to a ceasefire in May last year after Tuareg and Arab rebels claimed control of Kidal and other northern towns, dealing a humiliating defeat to the Malian army.