Diplomats in Algiers have expressed fears for the safety of 11 European tourists missing in the Sahara Desert since 21 February.
Tour firms offer off-road biking holidays in the Sahara
Algerian searchers have found no trace of a group of six German and one Dutch motorcyclists, and a second group of four Swiss travelling in a four wheel drive vehicle, who were holidaying near the Libyan border.
Numerous bands of smugglers, some with links to Islamic militants, operate in the region which is a vast natural wilderness.
"It leaves us baffled and we are forced to consider all eventualities, even that they have been kidnapped even though for the time being we don't know why," one unnamed diplomat told AFP news agency.
Helicopters equipped with heat-seeking technology - capable of locating bodies and machines buried under sand - have failed to locate any trace of the 11, who disappeared between the towns of Ouargla, Illizi and Djanet.
Their disappearance was first officially reported on 17 March by the Algerian news agency APS and there has been no official comment.
Embassies and travel agencies are said to be baffled by the tourists' fate.
The biker group had set out from Djanet, about 1,500 kilometres (930 miles) south-east of Algiers, and were due to travel north to Ouargla.
They were planning to travel via Illizi, a town about 100 kilometres (62 miles) north of Djanet.
The Swiss group had left Ouargla in the opposite direction, heading for Illizi, where the terrain is said to be particularly difficult.
"Eleven people with vehicles and all their stuff, that leaves traces," AFP's source said.
"They can't just vanish into thin air."
Some bands smuggling arms and drugs along Algeria's borders with Libya and Niger have ties with Islamic militants fighting the Algerian Government.
Algerian militants were blamed for disrupting the 2000 Paris-Dakar rally, which had to be re-routed away from the Niger-Algeria border after terror threats.