(Adds more details from Red Cross, background throughout)

By Abdoulaye Massalatchi

NIAMEY, June 28 (Reuters) - Nomadic rebels in northern Niger handed 30 wounded government soldiers over to the Red Cross on Thursday after taking them prisoner during a raid on a remote military outpost in the Sahara desert a week ago.

The Niger Movement for Justice (MNJ) killed 15 soldiers and took dozens hostage last Friday in a dawn attack on two army companies in Tazerzait, a remote settlement around a well in the Sahara desert near the Algerian and Malian borders.

Thirty of the most seriously wounded captives had been freed and handed to a Red Cross team east of Iferouane from where they were being taken to the larger town of Arlit for treatment, the International Committee of the Red Cross (ICRC) said.

"They hope to arrive later tonight. They can only move very slowly because they have injured people on board," Friedrun Medert, head of ICRC's regional office, told Reuters in Dakar.

"In Arlit we have a surgical team waiting for them who will operate on those requiring it during the night," she said, adding between three and six were thought to be gravely wounded.

Northern Niger has long been a hotbed of dissent, largely beyond government control, full of disillusioned, unemployed youths and awash with arms left over from an uprising by Tuareg, Arab and Toubou nomads in the 1990s.

Most of those rebel groups accepted peace deals in 1995 but the MNJ says the government has not lived up to its promises, failing to integrate former fighters and leaving the north economically marginalised and rife with insecurity.

It has stepped up attacks on military targets, raiding the airport of Agadez -- the north's main town popular with tourists -- this month in a bid to destroy military surveillance planes.

CALL TO NEGOTIATE

Medert said the ICRC convoy's security had been guaranteed by the Niger armed forces and the MNJ rebels during its journey of more than 100 km (62 miles) across the Air Massif, majestic mountains in the rebel heartland rising from the desert floor.

"The co-operation in security terms from the Niger army and from the MNJ has been excellent. My colleague leading the convoy feels safe and is confident that by tonight he will be able to bring the injured soldiers to Arlit," she said.

The MNJ says it launched its campaign against the army in retaliation for the arbitrary arrest and killing of civilians in the north during security clampdowns. It says 253 civilians have disappeared after a wave of detentions by the military.

"Our logic is not one of war. It is one of clear demands. We want justice for all of Niger's citizens," MNJ spokesman Seydou Kaocen Maiga told Reuters by telephone from Paris.

"The army should leave civilians alone. It should assure their security. If it does not ensure their security it should at least not kill them. Yet they are continuing to do so. We cannot stay indifferent to that," he said.

Niger's army has said it does not kill innocent civilians.

At least 33 soldiers have been killed in the north since the MNJ launched its campaign in February. But President Mamadou Tandja's government refuses to use the word rebellion, saying the attackers are drug traffickers and common criminals.

The former French colony's Democratic and Social Convention (CDS), the second biggest political party in the governing coalition, increased pressure on Tandja to negotiate late on Wednesday, calling on him to set up a team of mediators. (Additional reporting by Nick Tattersall in Dakar)

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