The Malmo midfielder was in the right place at the right time 51 minutes in to get to a rebound inside the box and rifle the ball past Danijel Subasic.
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Mario Mandzukic then came close to pulling level just moments later with a strike from the edge of the box which sailed just over the bar.
Berget, however, went on to double Norway's lead in the 70th minute following a skilful individual run finished off by a strike which deflected off Vedran Corluka and into the bottom-left corner.
The result means that Norway leapfrog to the top of Group H, with Croatia in second and Euro 2012 finalists Italy in third.