OSLO (AFP) - In an effort to solve its growing prison shortage, Norway has asked to rent prison space in neighbouring Sweden, the Norwegian justice ministry said on Thursday.

In a letter sent this week to his Swedish counterpart Beatrice Ask, Norway's Justice Minister Anders Anundsen explained that the situation in his country made it necessary to "look for extraordinary methods".

"I know that Sweden has vacant places and that some correctional facilities are closing down," he wrote.

"Thus, I want to raise the possibility of renting prison places."

About 1,200 people are currently waiting to serve sentences in Norway but are unable to due to lack of prison space, according to the justice ministry.

Swedish Prison and Probation figures show that the country's inmate population dropped by 1 per cent a year between 2004 and 2010, and by six per cent in 2011 and 2012, down to about 4,300 inmates today.

Some have attributed Sweden's diminishing prison population to the increased use of non-custodial sentences, including electronic tagging and community service.

Sweden has 82 prison facilities and closed down four of them and one rehabilitation centre in the last year.

Norwegian trade unions said Anundsen's initiative was understandable, but called it an "admission of failure".

"It's tragic that a country as rich as Norway must turn to its neighbour," Mr Geir Bjoerkli, head of the Norwegian Prison and Probation Officers Union, told Agence France-Presse, adding that the request could run into a number of legal and logistical pitfalls.

"The solution is simple: we need to start planning the construction of new prisons right away... and we also need to develop alternative sentences," he said.

Norway is not the first European country to look to a neighbour for help with overflowing prisons.

Since 2010 Belgium has rented an entire prison from its northern neighbour, the Netherlands.