Bamako - Mali's army launched airstrikes Thursday to halt the advance of Tuareg rebels amid calls for a ceasefire and as clashes sent thousands fleeing to neighbouring countries, military sources said.

Witnesses and army officials reported army helicopters were hitting targets in the north-east to prevent the rebels from moving towards Kidal, the region's biggest town.

UN chief Ban Ki-moon on Wednesday urged an end to the fighting as the Tuareg seized the town of Tinzawaten, after weeks of attacks on small desert towns which have seen more than 20,000 flee to neighbouring countries.

The Red Cross said Thursday that "at least 30,000 people are displaced in Mali and living in extremely precarious conditions."

The global humanitarian organisation said it had been able to visit 13 soldiers held "by an armed group" in northern Mali, without disclosing where they were being held.

This is the first indication of how many soldiers have been captured in a region where information cannot often not be independently verified.

The Azawad National Liberation Movement (MNLA) and other Tuareg rebels -- boosted by the return of men who fought in Libya for toppled leader Moamer Kadhafi -- are demanding greater autonomy for their nomadic desert tribes.

The MNLA was formed at the end of 2011, and on January 17 launched the first offensive since a Tuareg rebellion ended in 2009.