The Swiss authorities have put together a special team in an attempt to find four tourists who disappeared in the Algerian desert in February.
The missing Swiss are just one of six groups of European travellers that have vanished without a trace in Algeria in the past six weeks.
Altogether, 29 tourists from Switzerland, Germany, Austria, the Netherlands and Sweden have gone missing in the same area of the Sahara Desert.
All set off at different times and from different places by themselves. Most of them apparently disappeared either near the Libyan border close to the town of Illizi or not far from the southern Algerian town of Tamanrasset.
A crisis team set up by the Swiss has been coordinating with the Germans and the Austrians as the Algerian authorities continue to search for the missing tourists.
On Wednesday, two Swiss federal police officers arrived in Algeria to help with investigations. They are to work with three members of the Swiss embassy who have been following the case.
The Swiss foreign minister, Micheline Calmy-Rey, spoke to her Algerian counterpart about the search on Wednesday.
The Swiss authorities say they are prepared to take further steps, if necessary.
"The pressure is on the Algerians," foreign ministry spokeswoman Daniela Stoffel told swissinfo. "We have made it very clear at the highest levels how important this is for us."
The ministry said it did not consider the disappearances to be random events, but that they had a systematic character.
The German and Austrian authorities have also sent anti-terrorism specialists to Algeria.
Algeria has mounted a huge search operation, with rescuers scouring the desert on camels, with four-wheel-drive vehicles, and in aircraft and helicopters equipped with heat-seeking sensors. The Swiss foreign ministry says around 600 people have taken part in the search.
"Algeria has been doing everything it can for the past three weeks to find the tourists," said Amar Ben Salem, a correspondent for Radio France International.
"The task isn't easy though," he told swissinfo. "It's an area that's bigger than France that has to be searched."
The four Swiss did not have a guide when they set off into the desert, but the group's leader, Marc Hediger, had travelled there a number of times previously.
There has been no news of the tourists since February 20, when they made their last telephone call, and no sightings of them, their luggage or their vehicles.
So far the search has failed to turn up any results, and no hypothesis can be ruled out, according to the foreign ministry. "We haven't had any hint of where these people might be," said Stoffel.
No ransom demands have been made and no one has claimed responsibility for a possible kidnapping.
There has been speculation that the tourists may have been the victims of Islamic militants, who are waging a civil war against the government. Some of these militants have been linked to the al Qaeda network of the Saudi dissident, Osama bin Laden.
However, the foreign ministry doesn't believe that Islamic militants are behind the tourists' disappearance. "This is far-fetched," Stoffel told swissinfo.
Local media have also suggested that drugs and arms smugglers who are rife in the region may be responsible.
The foreign ministry recommends people avoid travelling to Algeria because of the civil unrest there. But despite the potential dangers that threaten tourists, the area appeals to foreigners.
The French have been travelling there in large numbers since a direct flight to Tamanrasset from Paris was introduced. Travel agents say people go to the region especially to see the mountainous Hoggar landscape.
swissinfo, Scott Capper and Vanessa Mock
The four Swiss tourists left Switzerland on February 8, planning to return on March 8.
The Swiss authorities were warned of their disappearance on March 12.
The Swiss embassy in Algiers has already taken part in a number of searches for the lost tourists, alongside the Algerians, Germans and Austrians.
Since February, other groups of tourists have vanished in the same area of the Sahara Desert.
In total 15 Germans, 8 Austrians, four Swiss, one Dutchman and one Swede have disappeared.
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