BAMAKO, June 5 (Reuters) - Mali's armed forces are engaged in heavy fighting with Tuareg rebels after attacking their positions in the northeast Kidal region, and both sides had suffered casualties, a senior Malian officer said on Thursday.

The clashes took place around the Insalat stronghold of Tuareg insurgent leader Ibrahima Bahanga, whose nomadic fighters have been attacking army garrisons and convoys in the rugged northeast of the West African Sahel state since last year.

"This is violent combat with heavy weapons ... we don't have a casualty toll yet, but it's going to be heavy," the officer, who asked not to be named, told Reuters in Bamako.

He said that the army had gone on the offensive on Wednesday against Bahanga's forces at Insalat, around 100 km (60 miles), from the northeast Saharan trading town of Kidal.

The rebels, from nomadic communities who traditionally resist interference from outsiders and staged previous revolts in the 1960s and 1990s, counterattacked against government forces in the Tegargar sector, the officer added.

The fighting appeared to be the heaviest since an assault last month by a combined force of rebel bands against an army garrison in the Kidal region. The Malian defence ministry said 17 rebels and 15 army soldiers were killed in that raid.

In a sign the Malian conflict is escalating, close to 1,000 Tuareg civilians have fled south to Burkina Faso to seek refuge, and the Burkinabe government appealed on Thursday for international assistance to help care for them.

The latest fighting also coincided with a visit to Mali by a prominent United Nations official, Jan Egeland, special adviser on conflict resolution to the U.N. Secretary-General.

He flew later on Thursday to neighbouring Niger, where Tuareg-led insurgents have been waging a separate rebellion in the northern uranium-producing region of Agadez. (For full Reuters Africa coverage and to have your say on the top issues, visit: http://africa.reuters.com/) (Editing by Pascal Fletcher)

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