Smugglers and drug traffickers are known to haunt the region

The news came as the Algerian police's own inquiry appeared to be stalling, with no new clues reported in the last few days.

Eight Austrian tourists were the latest group to be reported missing last week, when they failed to board a ferry in Tunisia as scheduled.

Several groups including 16 Germans, four Swiss and one Dutch national, have disappeared since the end of February.

All have disappeared in a huge triangle stretching from Ouargla in the north to the towns of Tamanrasset and Djanet in the far south of Algeria.

Co-operation urged

The German criminal investigation agency (BKA) said five investigators had gone to Algeria to work with local authorities.

"Whether or not more BKA investigators are sent will depend on the situation we find ourselves in," a BKA spokesman told Reuters news agency.

"Until we have concrete findings, we will not speculate on the cause for the disappearance."

The Austrian Foreign Ministry has also sent two diplomats and two officers from a special police unit.

President Thomas Klestil wrote to his Algerian counterpart Abdelaziz Bouteflika asking him to "do his utmost" to find the missing.

Germany and Austria have issued travel warnings for Algeria, urging all their citizens to leave the country or contact their embassies.

Kidnap fears

Helicopters using heat-seeking devices - capable of locating bodies and machines buried under sand - have been enlisted to find those who have disappeared, so far without success.

Smugglers and drug traffickers are known to haunt the area around southern Algeria, near the borders with Niger and Libya, and there are fears the tourists may have been kidnapped.

The region has been relatively free of fundamentalist violence common to other parts of Algeria, but one group is known to operate there.

It is led by Mokhtar Belmokhtar, who is believed to have joined the Salafist Group for Preaching and Combat (GSPC) - a hardline insurgency with alleged links to al-Qaeda.

However authorities have said that the travellers could also have experienced vehicle problems because of sand and extreme temperatures.