Norway's vast oil fund saw a decline of 2.2 percent in the second quarter, losing 77 billion kroner ($13 billion) in the period, mainly because of investors' concerns over the global economy.

Fund senior manager Yngve Slyngstad says the development means the government's goal of an annual 4 percent return on the fund is unlikely to be achieved in the near future.

Slyngstad said Friday that the target could be reached if there was a positive development in the world economy during 10 to 20 years, but that short-term propspects were "difficult."

Norway uses the fund to invest income from its offshore oil and gas sector.