(Adds further comment, discovery of torture unit)
* 200-250 military vehicles in Libyan convoy
* Gold, looted cash taken to Niger --Libyan council
* Sources say Gaddafi could have struck deal
* Fallen strongman may be headed to Burkina Faso
By Emma Farge and Abdoulaye Massalatchi
BENGHAZI, Libya/AGADEZ, Niger, Sept 6 (Reuters) - Scores of Libyan army vehicles crossed the desert frontier into Niger in what may be a bid by Muammar Gaddafi to seek refuge in a friendly African state, military sources from France and Niger told Reuters on Tuesday.
The Libyan rebels who overthrew Gaddafi two weeks ago said they also thought about a dozen other vehicles that crossed the remote border may have carried gold and cash apparently looted from a branch of Libya's central bank in Gaddafi's home town.
Details of the developments remained very sketchy.
The military sources said a convoy of between 200 and 250 vehicles was escorted to the northern city of Agadez by the army of Niger, a poor and landlocked former French colony. It might, said a French military source, be joined by Gaddafi en route to adjacent Burkina Faso, which has offered him asylum.
U.S. officials said they thought Gaddafi was still in Libya, though the convoy in Niger might contain senior figures.
France, Niger and Burkina Faso, as well as Libya's new rulers and NATO, all denied knowing where Gaddafi was or of any deal to let him go abroad or find refuge from Libyans and the International Criminal Court who want to put him on trial.
French Foreign Ministry spokesman Bernard Valero said it was for Libyans to decide the venue but that Gaddafi must not slip away quietly. "He will have to face justice for all the crimes he has committed in the past 42 years," he said.
Near Tripoli, Reuters journalists found torture chambers used recently as Gaddafi tried to suppress the revolt.
Sources close to Niger's government said the head of Gaddafi's security brigade, Mansour Dhao, was in the capital Niamey. He was allowed in to the country earlier in the week.
But Niger's foreign minister, Bazoum Mohamed, was quoted by Al Arabiya television saying that Gaddafi was not in the military convoy, which arrived late on Monday.
Those comments did not contradict a French military source who said the 69-year-old fugitive and his son and heir Saif al-Islam might join the convoy later to head for Burkina Faso.
France has taken a lead in the NATO action backing Libya's uprising and, with its Western allies, would be likely to have the ability to track any sizeable convoy in the empty quarter.
But Niger's ambassador to the United Nations in Geneva, Adani Illo, told Reuters that such surveillance over thousands of miles of desert was still hard. "The desert zone is vast and the frontier is porous," he said. "If a convoy of 200 to 250 vehicles went through, it is like a drop of water in an ocean."
Gaddafi has broadcast defiant messages since he was forced into hiding two weeks ago, and has vowed to die fighting on his own soil. But he also has long friendships with his poor African neighbours, with which he shared some of Libya's oil wealth.
The sources said the convoy, probably including officers from army units based in the south of Libya, may have looped through Algeria rather than cross the Libya-Niger frontier. Algeria last week took in Gaddafi's wife, daughter and two other sons, angering the interim council now ruling Libya.
Gaddafi's fugitive spokesman Moussa Ibrahim said in remarks broadcast on Monday: "Muammar Gaddafi is in excellent health and in very, very high spirits ... He is in a place that will not be reached by those fractious groups, and he is in Libya."
NATO warplanes and spy satellites have been scouring Libya's deserts for months, raising the likelihood that any convoy of the size mentioned would have been spotted. But a spokesman for the Western alliance said it was not hunting Gaddafi and had a U.N. mandate only to stop his forces attacking civilians.
"Or mission is to protect the civilian population in Libya, not to track and target thousands of fleeing former regime leaders, mercenaries, military commanders and internally displaced people," Colonel Roland Lavoie said in a statement.
Tuareg nomads living in the Sahara say those fleeing Libya include many black Africans, some of whom may have been fighters for Gaddafi and most of whom fear the anger and reprisals of Gaddafi's enemies among Libya's Arabs.
NTC commanders last week said both Gaddafi and his son Saif al-Islam were in the tribal stronghold of Bani Walid, 150 km (90 miles) south of Tripoli. But that belief has evaporated this week after days of blockade of the town.
NTC officials said Saif al-Islam, for one, may have escaped south into the desert, toward the southern, pro-Gaddafi bastion of Sabha and perhaps on to Niger. Tracking him would be hard; fully 1,300 km (800 miles) of sand separate Sabha from Agadez, with a further 750 km of road to travel to Niamey.
Near Sirte, Gaddafi's home town on the Mediterranean coast, there was the first sign of heavy fighting for some days. Combatants reported exchanges of shell fire and rockets to the east. Several NTC fighters were wounded in an ambush.
Though conditions in Tripoli were improving with the return of water supplies two weeks after rebels overran Gaddafi's headquarters compound, evidence of brutality during his battle to cling to power during the Arab Spring is also accumulating.
Reuters journalists in the provincial town of Khoms found evidence Muammar Gaddafi had deployed squads which held suspected opponents in shipping containers, tortured them for information about insurgent networks and disposed of their bodies in unmarked graves.
"They wanted to frighten the people, so if anyone was thinking of going over to the rebels, they would change their minds," said Nabil al-Menshaz, a council official in the town.
A spokesman for the NTC said banknotes in the convoy of gold and cash that the council believed had reached Niger had been stolen from Sirte's branch of the Central Bank of Libya.
NTC official Fathis Baja told Reuters: "Late last night, 10 vehicles carrying gold, euros and dollars crossed from Jufra into Niger with the help of Tuaregs from the Niger tribe."
It was unclear if these vehicles were separate from the much larger military convoy reported by the foreign sources.
Burkina Faso, also once a French colony and a recipient of large amounts of Libyan aid, offered Gaddafi sanctuary last month but has also recognised the NTC as Libya's government.
President Blaise Compaore, like Gaddafi, took power in a military coup. He has run the country for 24 years.
Gaddafi has long touted his origins among the peoples of the desert. After largely turning his back on fellow Arab leaders, most of them allied with his Western adversaries, Gaddafi had portrayed himself as an African "king of kings".
He was fond of epic road journeys on his travels around the continent, so the drama of a flight across the Sahara into friendlier lands further south might seem a fitting departure. (Reporting by Mohammed Abbas, Christian Lowe and Alex Dziadosz in Tripoli, Sherine El Madany in Umm Qundil, Marie-Louise Gumuchian, Barry Malone and Alastair Macdonald in Tunis, Sami Aboudi, Amena Bakr and Omar Fahmy in Cairo, Erika Solomon in Dubai, Nathalie Prevost in Niamey, Stephanie Nebehay in Geneva, David Brunnstrom in Brussels and Richard Valdmanis and Mark John in Dakar; Writing by Alastair Macdonald)