* Foreign minister calls for disarming of rebels
* U.N. fears a return of war in the north
* France plans to send 3,000 to fight Islamists in Sahel
* Delayed to concentrate on new fighting in north Mali (Recasts with foreign minister's comments, adds quotes In paragraphs 4, 7, 11-12, and details)
By Adama Diarra and John Irish
BAMAKO/PARIS, May 20 (Reuters) - Mali's government on Tuesday called for an international commission to investigate a weekend attack by Tuareg separatist rebels that has revived fears of a return to war in the West African nation's troubled north.
The rebels killed eight Malian soldiers in clashes in the northern town of Kidal on Saturday and also killed eight civilians, including six government officials, when they attacked the regional governor's office there. The violence took place during a visit by Prime Minister Moussa Mara.
The army retook key positions in Kidal, the traditional stronghold of the separatist National Movement for the Liberation of Azawad (MNLA), on Tuesday without a fight. But the separatists remained in control of a military barracks and the governor's office, a military source and a witness said.
"The act of aggression that occurred in Kidal cannot remain unpunished," Foreign Minister Abdoulaye Diop told the United Nations Security Council during a videoconference. "We ask that an international commission be created to verify the facts in order that those responsible can be brought before national and international justice."
Diop also asked for the United Nations support in disarming the rebels, who have remained armed under a preliminary peace deal signed last year.
Mara initially called the attack a "declaration of war" and the Malian army sent troop reinforcements to Kidal. However, President Ibrahim Boubacar Keita said late on Monday that the government remained open to dialogue.
"All the (army) reinforcements have arrived," a senior military source told Reuters on Tuesday. "We are just waiting for a political decision before moving onto the offensive."
Mali's international partners have been pushing for a final, negotiated settlement to a long cycle of Tuareg independence uprisings since al Qaeda-linked fighters hijacked a Tuareg rebellion in 2012 and seized the country's desert north.
After a French-led intervention drove the Islamists from major cities and towns last year, the government and separatist groups agreed to hold talks over greater autonomy for the north.
But the lines between the independence fighters and the Islamists remain blurred, and it has been difficult to get the government and separatists to sit down together.
"The priority today is to pull Kidal back from the brink of renewed confrontation," Albert Koenders, the head of Mali's U.N. peacekeeping mission MINUSMA, told the Security Council.
"It is essential to prevent northern Mali from slipping into a spiral of violence that risks drawing Mali back to the state of crisis that ... could destabilize the entire sub-region."
MINUSMA is still not at its full strength of 13,000, and while it was present in Kidal on Saturday it was unable to stop the fighting.
France, which still has around 1,600 soldier in Mali, had hoped to reduce its troop numbers there as part of a plan to redeploy 3,000 soldiers to fight Islamist groups operating between southern Libya, northern Chad and northern Niger.
It fears the fighters could use the region as a base for wider attacks.
But officials said on Tuesday that the plan had been delayed.
"Given the events of the last 48 hours, the operation to transfer operation Serval (in Mali) to a Sahel-Sahara French force must be delayed for several weeks," a Defence Ministry source said on Tuesday.
A French military source said no new date had been set for the broader West African deployment, originally scheduled to be completed by the end of May.
Defence Minister Jean-Yves Le Drian postponed a May 25 trip to Mali and Chad, where the operation will be based.
The French army said it now had about 90 soldiers in Kidal, having sent an extra 30 as a precautionary measure.
"There is an urgency in the coming days to restore calm in Kidal, because this must not derail the reconciliation process," a French diplomatic source said. (Additional reporting by Marine Pennetier in Paris, Tiemoko Diallo in Bamako, Emma Farge in Dakar and Michelle Nichols at the United Nations; Writing by John Irish, Matthew Mpoke Bigg and Joe Bavier; Editing by Mohammad Zargham)