* Army forces, Tuareg rebels clashed in Kidal at weekend
* Army reinforced lines before expected offensive
* Rebels call for ceasefire
* Region also a haven for Islamist militants
* Ex-colonial power France sends more troops (Adds government's statement)
By Adama Diarra and Tiemoko Diallo
BAMAKO, May 21 (Reuters) - Mali has launched a military offensive on Wednesday to retake control of a northern Tuareg separatist stronghold, the government said, and witnesses reported intense fighting with machine gun and heavy weapons fire.
The clashes threaten efforts to find a peaceful solution to the long cycle of Tuareg rebellions in the desert north. It also upsets plans by France and several West African countries to combat Islamist militants operating elsewhere in the region.
Malian soldiers and Tuareg separatists clashed on Saturday while Prime Minister Moussa Mara was visiting the town of Kidal. At least eight soldiers and eight civilians were killed.
The army had been reinforcing its positions since then, in preparation for an expected campaign to retake Kidal.
"At around 10 a.m., the Malian armed forces launched operations to secure and take control of Kidal. The operations are ongoing," a government statement read on state radio said.
A Defence Ministry source said the army had begun an assault on the regional governor's office in Kidal after it was seized at the weekend by Tuaregs from the National Movement for the Liberation of Azawad (MNLA).
"The combat will continue until we achieve the total liberation of the town," the source said.
Spokesmen for the ministry and Mali's U.N. peacekeeping mission confirmed 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 by telephone.
A Kidal trader told Reuters that the town's main market had been destroyed in the fighting.
Attaye Ag Mohamed, an MNLA official in Kidal, accused the Malian army of starting the clashes and called on 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 said by telephone. Blasts were 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 occurred as West African nations and their international partners have been redoubling efforts to contain Islamist insurgents such as Boko Haram in Nigeria.
France, 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, 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.
"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. Numbers were being increased for two to three weeks to allow "more flexibility", he said. (Additional reporting by Bate Felix in Dakar and Marine Pennetier in Paris; Writing by Joe Bavier; Editing by Mark Heinrich)