Counter-terrorism experts differ as to how the attackers - in several pickup trucks - may have reached In Amenas, but there are several roads and tracks across uninhabited desert from Libya.

CNN: STORY HIGHLIGHTS

Some of the tribes there have a history of hostility to the colonel, while the rugged Tuareg people who populate a swathe of the Sahara desert from Libya to Mali, and were long favoured by the Libyan leader with jobs and guns, seem to be abandoning him.

ECONOMIST: Libya

Most incidents over the past two years have involved armed groups using Tunisia's southern desert to pass between Algeria and Libya.

NPR: Jihadi Peril Makes Its Way To Tunisia

The crisis began on 16 January when militants attacked two buses carrying foreign workers from the In Amenas gas facility, which lies in the desert in eastern Algeria, not far from the border with Libya.

BBC: Algeria siege dead and survivors flown back to Japan

Mr. Shakmak said he had no knowledge of a particular threat to Libya, but said Tripoli was sending more troops to the border and to desert oil fields, and boosting communications capacity, as a precautionary measure in response to the Algeria attack.

WSJ: Libya Boosts Oil-Field Security

Libya has a long coastline on the Mediterranean but the greater part consists of the central Sahara Desert.

BBC: Libya

The best is in the Sahara desert, particularly in Niger's Air mountains, in the Tibesti mountains of northern Chad and southern Libya, and in south-east Algeria's Tassili n'Ajjer range.

ECONOMIST: African rock art