* Government forces, rebels clashed over weekend
* Army reinforced positions before expected offensive
* Rebels call for ceasefire
* France sends additional troops (Adds comments from defence ministry source, MNLA official, French military spokesman)
By Adama Diarra and Tiemoko Diallo
BAMAKO, May 21 (Reuters) - Machine gun and heavy weapons fire broke out on Wednesday in the northern Mali town of Kidal, a rebel stronghold where the army has been reinforcing its positions before an expected drive to retake it, an army officer and the United Nations said.
The renewed violence follows clashes between the army and Tuareg separatists that broke out on Saturday while Prime Minister Moussa Mara was visiting the town, killing at least eight soldiers and another eight civilians.
The clashes threaten efforts to find a peaceful solution to the long cycle of Tuareg rebellions in Mali's desert north. It also upsets plans by France and several West African countries to combat Islamist groups operating elsewhere in the region.
A Malian Defence Ministry source said the army had begun an assault on the regional governor's office in Kidal. Tuareg separatists from the National Movement for the Liberation of Azawad (MNLA) had seized the office over the weekend.
"The combat will continue until we achieve the total liberation of the town," the source said.
Spokesmen for Mali's Defence Ministry and the country's United Nations peace-keeping mission both confirmed that fighting had restarted but declined to give further details.
"These aren't just shots, it's fighting. There's been shooting for an hour without interruption," Kidal resident Assikadaye Ag Warzagane told Reuters.
Attaye Ag Mohamed, an MNLA official in Kidal, accused the Malian army of starting the clashes and called upon the U.N. mission and international community to press for a ceasefire.
"The Malian army launched an offensive, opening up with heavy weapons at 10 o'clock (1000 GMT) this morning ... Right now intense heavy weapons and machine gun fire is continuing," Mohamed told Reuters by telephone, as explosions could be heard in the background.
Mali was thrown into turmoil in 2012 when al Qaeda-linked Islamists took advantage of a Tuareg-led rebellion and seized control of the country's north. A French-led military operation, known as Serval, drove them back last year.
The government agreed with a number of armed groups to hold talks over autonomy last year. The groups included the MNLA, which broke with the Islamists before the French offensive.
The flare-up in a trouble spot many had hoped was now under control came as West African nations and their international partners redoubled efforts to contain Islamist groups such as Boko Haram in Nigeria.
Paris, in particular, had hoped to move troops from Mali and other bases to target Islamist groups operating between southern Libya, northern Chad and northern Niger, fearing the fighters could use the region as a base for wider attacks.
However, French officials said on Tuesday the redeployment, which is expected to involve around 3,000 troops, would be delayed in light of the situation in northern Mali.
And on Wednesday, an army spokesman said France would reinforce its presence in Mali with 100 more soldiers, bringing total deployment to its former colony to some 1,700 troops.
"The decision has been taken to bring forces from Abidjan to Gao, given the current period of tension," spokesman Gilles Jaron told reporters in Paris.
The numbers were being increased for two to three weeks in order to allow "more flexibility" to the French force, he said. (Additional reporting by Bate Felix in Dakar and Marine Pennetier in Paris; Writing by Joe Bavier; Editing by Larry King)