Bamako - Mali's UN peacekeeping mission deployed troops around a northern separatist stronghold on Tuesday, seeking to prevent an escalation of clashes between rebels and pro-government militias that threaten to torpedo a peace deal.The separatists, led by Tuareg tribesmen, and the pro-government Platform militias signed the peace accord in June. The aim was to pacify the north and allow the Malian army to focus on tackling Islamist militants in the west African country.The two sides have traded blame for the clashes, which began on Saturday. On Monday, Platform fighters seized the town of Anefis from the separatist Co-ordination of Azawad Movements (CMA), raising fears they would advance on the town of Kidal, the group's main stronghold."These acts constitute a flagrant violation of the ceasefire and the peace accord," the UN mission, Minusma, said in a statement.Minusma called on both sides to immediately return to the positions they occupied on August 15, and said it was putting in place a security zone extending 20km around Kidal to prevent a spread of the violence and protect civilians."In the case of a violation of this security zone by Platform elements or those affiliated with the Platform, Minusma will react in accordance with its mandate," the mission said.There are over 9 000 UN peacekeepers in Mali, 90% of them in the north. The UN force did not specify how many troops were deployed around Kidal.No clashes were reported on Tuesday. A Reuters witness said CMA fighters were reinforcing positions around Kidal and deploying heavy weapons.Don't understandFahad Ag Almahamoud, the secretary general of the GATIA pro-government militia, said the UN mission's action had handed part of Mali's territory to a rebel group."We do not understand this decision by Minusma," he said.Mohamoud Ag Ghaly, secretary to the president of the CMA, said the group had no need of UN protection and had already repulsed an effort to take Kidal.He said the UN mission was failing to protect civilians in northern towns occupied by the militia and the Malian army, including Menaka and Anefis.Neighbouring Niger was due to begin hosting talks on Wednesday to ease tensions between the two sides.Mali says it has asked the African Union and the United Nations to investigate the ceasefire violations. Minusma on Monday threatened to apply targeted sanctions against those found to be behind the violence.The West African nation is seeking to break a decades-long cycle of Tuareg uprisings, the most recent of which allowed Islamist groups, some linked to al-Qaeda, to seize the desert north in 2012.A French-led intervention a year later scattered the Islamists but failed to eradicate them, and Islamist violence is once more on the rise and expanding further south.