* Aid urgently needed for one of earth's poorest regions

* U.N. prefers dialogue to solve Tuareg conflicts

* Security measures required to tackle trade in drugs, arms

By Pascal Fletcher

DAKAR, June 5 (Reuters) - Climate change, fighting over water and land, trafficking in drugs, arms and migrants, and the grievances of nomadic Tuaregs have created a "lethal cocktail" threatening Africa's Sahel belt, a top U.N. official said.

Since last year, insurgencies have flared among the Tuareg communities of northern Mali and Niger, stirring up an already volatile region where millions face grinding poverty as drought and the Sahara desert degrade their environment.

Its name derived from the Arabic "sahil" meaning shore, the Sahel is a belt of largely barren sand and rock that runs from the Atlantic Ocean to the Red Sea and separates Africa's vast Sahara desert from the more fertile lands to the south.

"There is no place where there is such a well-deserved need for international assistance, in my view, as these poorest countries on earth," said Jan Egeland, a special adviser on conflict resolution to the United Nations Secretary-General.

Egeland, a former U.N. humanitarian affairs chief, is visiting the Sahel states of Burkina Faso, Mali and Niger to focus attention on the pressures facing the region he calls "the front line of humanity's battles against climate change".

"This is precisely where climate change, environmental degradation ... resource conflict ... trafficking of drugs, arms and humans ... all come together in one lethal cocktail," he told Reuters in a phone interview from Mali late on Wednesday.

In Mali, Egeland visited dried-up lakes and watercourses and met Tuareg community leaders who complained of being neglected by their own nation and the world community and of feeling that their nomadic, pastoralist way of life was under threat.

With arms and drugs smugglers channelling large quantities of weapons into the Sahel, these grievances have fuelled the latest Tuareg revolt in Mali, in which nomadic fighters have attacked army camps and columns in the northeast Kidal region.

"These grievances connected to the worsened environment, on top of a sense of being marginalised ... feeling threatened as pastoralists, all of that may play up to young people feeling that 'no, I don't want more dialogue, I want to fight because dialogue brings nothing'" Egeland said.

FRESH FIGHTING REPORTED

Shortly after the U.N. official left Mali on Thursday to fly to neighbouring Niger, the Malian army reported fierce clashes in the northeast between its troops and Tuareg fighters. Casualties were heavy, it said, without giving details.

Last month, 17 Tuareg rebels and 15 government soldiers were killed in a rebel raid on an army camp in northeastern Mali. A Tuareg-led rebellion centred on the uranium-producing north of neighbouring Niger has killed more than 70 government troops.

In a sign the Malian conflict may be 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.

Egeland said he met Malian President Amadou Toumani Toure in Bamako and expressed the U.N.'s preference for dialogue, rather than a purely military response, to settle conflicts.

"The President of Mali ... wants dialogue, he recognises that the pastoralists have to be heard," Egeland said.

But he added that at the same time security measures were needed to try to stem the increase in criminal activity and banditry across the Sahel, especially drug-trafficking.

"There are Colombian drug lords now basically buying their way across the Sahara and they are causing conflict as well ... There has to be a regional law enforcement response," he said.

Colombian cocaine cartels have homed in on West Africa as a transit hub to ship drugs to Europe, both along the Atlantic coast and in desert convoys up through the Sahel and Sahara.

Regional governments and analysts are divided over the extent to which the Tuareg revolts in Niger and Mali are driven by genuine political grievances or whether they are fighting to control drugs, arms and migrant smuggling routes. (For full Reuters Africa coverage and to have your say on the top issues, visit: http://africa.reuters.com/) (Additional reporting by Mathieu Bonkoungou in Ouagadougou and Tiemoko Diallo in Bamako; Editing by Giles Elgood)

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