Ole Johan Grimsgaard-Ofstad's last communicated via his Facebook page on Jan. 24, announcing he had "finally made it" to Syria and was on his way to Hama. His prolific posting subsequently stopped.
Norwegian Foreign Minister Boerge Brende confirmed Thursday that the 48-year-old had been held since January, and that a picture shown in the latest issue of the militants' online magazine Dabiq, showing Grimsgaard-Ofstad in a yellow jumpsuit was believed to be recent.
The magazine lists a telegram number for "whoever would like to pay the ransom for his release and transfer."
"Based on our analysis of the image published, and the information we have, it is our assessment that the photo was taken during the last month," said Brende. "We are working on the basis that he is still alive."
Norwegian officials dismissed questions about why Grimsgaard-Ofstad went to Syria, saying it was not for them to speculate on the reasons.
On Wednesday, Norwegian Prime Minister, Erna Solberg said Norway "cannot and will not succumb to pressure from terrorists and criminals. Norway does not pay ransoms."
The IS magazine also showed an image of another hostage reported to be Chinese national, Fan Jinghui.
Grimsgaard-Ofstad, from Porsgrunn, south of Oslo, showed an interest in international affairs.
Marit Reitan, dean of the School of Social Sciences and Technology Management at NTNU in Trondheim, said that the political science graduate had enrolled for a post graduate course in international political philosophy last fall but had been removed from course in July this year when his fees went unpaid.
His Facebook history shows almost daily updates about Middle East conflicts, with a particular focus on Syria. Earlier posts also show an interest in Norse mythology.
Images posted on Ofstad's Facebook page in January appear to show him on the border of Turkey and Syria. Days later his final post said: "I am in Idlib, Syria. Going to Hama tomorrow. I finally made it."