A Boeing 737 that belongs to the SAS airline and which has been converted into a medical plane left Norway on Thursday but had been delayed in Sicily without permission to land near the In Amenas gas plant. It was finally expected to reach Algiers on Friday afternoon.

"For the moment, we haven't been authorized to land at In Amenas," foreign ministry spokesman Frode Andersen told AFP as the plane was still on its way to the Algerian capital.

On Friday, Oslo also dispatched another air ambulance, a C-130 Hercules military aircraft, to Sicily.

"The situation (at In Amenas) is so confusing that we would rather have planes in the region that can be quickly mobilised," said Bent-Ivan Myhre, a military spokesman.

The aircraft may be sent to the "most pertinent" location once it has received authorization to land, the foreign ministry said.

Norwegian authorities stressed that sending the planes was only a precautionary measure, and not in any way an indication of the fate of the eight Norwegians "affected" by the hostage-taking.

Asked about the nature of Norway's contacts with Algeria, Andersen said the two countries were in "close contact" but refused to provide any details.

The eight Norwegians are employees of the oil group Statoil, which jointly operates the gas field with BP and state-run Algerian energy firm Sonatrach. 

Islamists were still holding an undetermined number of foreign hostages at the isolated complex on Friday as criticism mounted of Algiers' decision to launch a deadly rescue bid at the site.