By Henrik Stolen and Nerijus Adomaitis
GARDERMOEN, Norway (Reuters) - Oil-rich Norway is stretching out its purchase of 52 F-35 Joint Strike Fighters from Lockheed Martin <LMT.N>, the Defence Ministry said on Friday, a move that will nudge up the total cost but reduce the annual burden on the defence budget.
The country is about to place firm orders for six of the fighter planes and will extend the timeframe for the purchase of all 52 from four years to eight years, the ministry said.
"We have looked into how we should acquire these aircraft through the most favourable time frame, so that we are able to phase out the F-16s and phase in the F-35 with pilots, equipment and everything else needed", Minister of Defence Anne-Grete Stroem-Erichsen told Reuters.
The F-35 is designed to be the next-generation, radar-evading fighter for U.S. forces and their allies, but costs have risen sharply and production has been delayed, leading some countries to scale down or rethink their orders.
The ministry said it had asked parliament to approve the purchase of six planes to be delivered in 2017, one year earlier than previously planned, but it would also take longer to complete the remaining purchases.
That means Norway, which formally ordered the first four aircraft last June, will complete the purchases during the period 2017-2024, instead of the original plan of 2018-2021.
Extending the period for the biggest weapons procurement ever for the Nordic country will bring the programme's total cost to 62.6 billion crowns (6.15 billion pounds) in 2013 crowns rather than 60 billion crowns (5.9 billion pounds) in 2012 terms.
But stretching out the period will lessen a burden on the annual defence budget. Norway will buy six planes each year for eight years, instead of 12 planes each year for four years.
Lockheed welcomed the news as a vote of confidence in the F-35 program and said it would result in "long term economic benefits" for Norway given the participation of Norwegian companies in planned jet production.
"We will work closely with the government to accommodate Norway's decision and support their plan to continue purchasing aircraft beyond the first four training aircraft funded in 2012," said Lockheed spokesman Michael Rein.
The first four F-35s ordered last year are expected to stay in the United States for training purposes, and the first new fighter planes out of the latest order are due to arrive in Norway in 2017.
In total, the U.S. military and its allies plan to buy over 3,100 F-35 aircraft.
(Reporting by Henrik Stolen and Nerijus Adomaitis; Additional reporting by Andrea Shalal-Esa in Washington; Editing by Mark Potter and David Gregorio)