* Gunfire rings out across northern town hours after raid
* Resident says hunt for attackers on-going
* Bombing underscores lack of security as U.N. force deploys (Adds shooting, details)
By Adama Diarra
BAMAKO, Oct 23 (Reuters) - A suicide bomber killed two Chadian troops from the U.N. peacekeeping mission in Mali and injured six others in an attack on a checkpoint at the entry to the northern town of Tessalit on Wednesday, a U.N. spokesman said.
Hours after the late-morning raid, gunfire rang out across the desert town as Chadian and Malian troops hunted down other attackers believed to have been dropped off before the explosion, officials said.
No group has claimed the attack. Al Qaeda-linked Islamists, who occupied northern Mali last year, have carried out a number of suicide attacks on northern towns in recent months since they were scattered by a French offensive launched in January.
Olivier Salgado, spokesman for the U.N. mission that is deploying in Mali to take over security responsibilities from French troops, said the bomber drove an explosive-laden vehicle to the checkpoint before blowing himself up.
"The provisional toll is two dead and six injured," he said.
Cheick Fantamady Bouare, the local prefect, said four people had been killed in total, including a child. The bomber was believed to have ferried other fighters to the town's outskirts before detonating his charge, Bouare said.
Another U.N. official, who asked not to be named, said gunfire continued into the afternoon.
Ibrahim Ag Mohamed, a Tessalit resident currently in Bamako but in touch with people on the ground, said that several buildings had been damaged and many people, including civilians, had been injured.
"The hunt for the attackers by the Malian, Chadian and French soldiers is continuing," he said.
A spokesman for Serval, France's military force in Mali, said French aircraft had been sent to evacuate the wounded and some troops were embedded as liaison officers with Chadian troops in the town. No French troops were injured, he added.
The United Nations appealed earlier this month for more troops and helicopters for the Mali mission, which is operating at half its authorised strength of 12,600 and has seen already seen some Nigerian troops withdraw.
Although the French operations swiftly defeated the Islamists who overran Mali's army last year, experts have warned that many may simply have gone to ground, regrouped and sought opportunities to strike as France reduces it presence.
Bert Koenders, head of the U.N. mission, said attacks late last month and in early October were a "wake-up call" to the challenges remaining.
Mali imploded last year when Tuareg separatist rebels took up arms in the desert north, triggering a coup by soldiers angry over the government's handling of the rebellion.
Islamists took advantage of the chaos and seized the north, stoking fears in the West that the zone would become a launch pad for militant attacks and a safehaven for criminal gangs operating alongside the various rebel groups.
Ibrahim Boubacar Keita, Mali's new president, must juggle rebuilding the shattered army and national pride while seeking to secure lasting peace with the Tuareg rebels who did not join the Islamists and are seeking a political deal. (Additional reporting by Tiemoko Diallo in Bamako; Writing and additional reporting by David Lewis; Editing by Alison Williams)