By Cheick Diouara

GAO, Mali (Reuters) - A suicide bomber killed three Chadian soldiers at a market in the northern Malian town of Kidal on Friday, a Chadian military official and three witnesses said.

Chadian and French soldiers have been stationed in the desert town since late January to fight Islamist rebels hiding in the region's mountainous borderlands.

"He blew himself up next to a group of Chadian soldiers. Three were killed in the blast and several others were wounded," resident Ag Waerzagane Assikadaye told Reuters by telephone.

The Chadian military official confirmed the death toll and added that four other soldiers were hurt.

Considered some of the best-trained troops in desert warfare and stationed in some of the most dangerous areas, the Chadian army has suffered the heaviest casualties of any of Mali's foreign allies and lost some 30 soldiers in fighting there.

Separately, Mali's Defense Ministry said in a news release on Friday that a Malian army helicopter crashed near the central town of Sevare, killing all five people on board. The cause of the crash was not immediately clear.

A wave of deadly bombings and raids by al Qaeda-linked Islamist fighters has raised fears that Bamako is failing to reassert its control in northern Mali's towns, after French forces drove the militants into desert and mountain hideouts.

Tuareg separatists, who say they are helping fight the Islamist insurgents, remain in power in Kidal, near the border with Algeria, complicating plans to reunite the West African country after last year's coup and northern revolt.

Mali's prime minister promised on Thursday that elections to replace the caretaker government would go ahead in July. But analysts fear preparations will not be completed in time and warn a botched poll could sow the seeds for further unrest.

Some 4,000 French troops are fighting alongside Mali's army and a regional African force, although Paris is aiming to cut its military presence to 1,000 soldiers by the end of the year.

Human Rights Watch said on Thursday that two Tuaregs suspected of backing the rebels and captured by Malian security forces in the central town of Lere in February died in Bamako's main prison after weeks of torture and mistreatment.

Mali's minister of justice, Malick Coulibaly, told reporters on Friday that three prisoners - a Moroccan and two Malians - had died, but he gave no other details and said their ethnicity was irrelevant.

(Reporting by Cheick Diouara, Adama Diarra, Tiemoko Diallo, and Madjiasra Nako; Writing by Richard Valdmanis; Editing by Jon Hemming and Peter Cooney)