* Army prepares to take back northern town from separatists
* Govt claims al Qaeda affiliate involved in weekend attack
* Rebels holding 30 civil servants hostage
* International partners push for restraint, dialogue (Adds Kidal official, rebel and French military spokesmen, analyst, details)
By Tiemoko Diallo
BAMAKO, May 19 (Reuters) - The United States warned on Monday that northern Mali risked sliding back into war and called for the government and Tuareg separatists to return to talks after deadly weekend clashes in a traditional rebel stronghold.
The Malian army was preparing to launch an assault on the northern town of Kidal where separatist fighters killed at least eight soldiers and took around 30 civil servants hostage in an attack on the regional governor's office on Saturday.
Eight civilians including six government officials were killed, according to the United Nations, during Saturday's assault which took place while the prime minister was visiting Kidal.
"We have convinced the head of state that it is highly desirable...that Kidal be totally under the control of the Malian state," Prime Minister Moussa Mara said in a televised address late on Sunday.
President Ibrahim Boubacar Keita was due to address the nation on Monday evening.
"The rebels and the army are reinforcing their positions," said an elected official in Kidal, who asked not to be named due to fear of reprisals. "I'm closed up in my house. The next hours will be decisive."
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 2012 rebellion and seized the West African nation's desert north.
After a French-led intervention drove the Islamists from major cities and towns last year, Mali's government and separatist groups signed a deal in Burkina Faso's capital Ouagadougou to hold talks over increased autonomy for the north.
But the lines between the independence fighters and their erstwhile Islamist allies remain blurred, and efforts to get the government and the separatists to sit down to negotiations have struggled.
"We are very concerned about what happened and that the response might lead to this region going back into conflict," U.S. Assistant Secretary of State for Africa Linda Thomas-Greenfield told journalists in Paris on Monday.
"It is important for the government to continue to talk to them and work on a reconciliation that will bring them back into the fold."
Kidal is the stronghold of the Tuareg National Movement for the Liberation of Azawad (MNLA) and while the rebels now claim to control the town, they said they had only fought back after coming under a Malian army attack.
Attaye Ag Mohamed, a spokesman for the group based in Kidal, reasserted on Monday that the rebels were prepared to hand over the civil servants. And he denied government accusations that the MNLA had renewed its previous alliance with armed Islamist groups.
"They can say what they want. They're just trying to dupe the Malian people," Ag Mohamed told Reuters.
MINUSMA, Mali's U.N. peacekeeping mission, called upon both sides to refrain from any violence that could endanger civilians in Kidal. However, the planned 13,000-strong force is not yet at full strength, and while it was present in Kidal on Saturday, it was unable to stop the fighting.
Meanwhile France condemned the killings of civilians and called for the immediate release of the remaining hostages.
However Paris, which led an offensive against armed Islamist groups but pressed Bamako to negotiate with the northern separatists, said it would stay out of any eventual fighting between the army and the Tuareg rebels.
"We are not there to intervene with regard to tensions between Malians," French army spokesman Colonel Gilles Jaron said. "We are 1,600 (soldiers) in Mali now and their role is to fight armed terrorist groups."
France is seeking to reduce its numbers in Mali to around 1,000 while redeploying soldiers elsewhere in the region to tackle the rising threat of Islamic militant groups that have spread across West Africa since last year's intervention.
"This is a real turning point in the crisis," Rinaldo Depagne, West Africa programs director for the conflict prevention think tank the International Crisis Group, told Reuters.
"It's clearly the end of the Ouagadougou agreement and the situation is as it was at the beginning of the crisis." (Additional reporting by John Irish and Marine Pennetier in Paris, Emma Farge in Dakar, Adama Diarra in Gao, and Joe Bavier in Abidjan; Writing by Joe Bavier; editing by Ralph Boulton)