Switzerland has stepped up its efforts to locate 15 western tourists - including four Swiss - who are missing in southern Algeria.
Officials said two additional experts have arrived in the capital, Algiers, to reinforce a Swiss team of investigators.
The fate of the tourists who disappeared more than two months ago is still unknown. They are apparently being held by Islamic militants in the Algerian desert.
The Swiss foreign minister, Micheline Calmy-Rey, urged her Algerian counterpart, Abdelaziz Belkhadem, to ensure they are freed unharmed.
On Tuesday, 17 other missing tourists - ten Austrians, six Germans and one Swede - were found alive and unharmed. Reports from Algeria said the tourists had been freed by security forces.
Those still missing are reportedly being held elsewhere by Islamic militants, who were said to have separated the 32 holidaymakers into groups.
The missing Swiss disappeared after heading into the Sahara desert without a guide last February.
"We do not know anything about what has happened to the four Swiss or their whereabouts at the moment," said a foreign ministry official.
Bern regretted that the other countries had publicly announced that their nationals had been found.
"We thought that [such announcements] could endanger the lives of the tourists who are still missing. We hope this is not the case, but indeed we are concerned," Muriel Berset Kohen, a spokeswoman for the foreign ministry, told swissinfo.
The foreign ministry said on Monday that it believed the four Swiss were still alive.
The Austrian and German governments have refused to comment on the circumstances of their nationals' release.
swissinfo with agencies
Switzerland has sent another two experts to Algeria to search for 15 western tourists who disappeared more than two months ago.
Four Swiss are among the missing holidaymakers, who are apparently being held by local Islamic militants in the south of the country.
On Tuesday, 17 other western tourists were freed by Algerian security forces.
Among them were ten Austrians, six Germans and one Swede.