By Adrian Croft
THE HAGUE (Reuters) - Former Norwegian Prime Minister Jens Stoltenberg has emerged as the favorite to take over the leadership of NATO later this year as the Western military alliance faces a challenge from a newly assertive Russia, diplomats said on Monday.
Several diplomats said Stoltenberg, who served for a total of nearly 10 years as Norway's prime minister before losing power in elections last September, has the backing of the United States, NATO's dominant power, and Germany. A number of other countries were also rallying around his candidacy.
"He's a pretty strong candidate," one NATO diplomat said.
Norwegian Prime Minister Erna Solberg said Norway had not officially put forward Stoltenberg as a candidate.
"We have seen that there (is) quite big speculation and a lot of countries would like him to stand as a candidate. But this is a consensus process," she told a news conference during a nuclear security summit in The Hague.
"Knowing NATO you can never be sure until it is done," she said.
The Norwegian daily Aftenposten reported on Monday that British Prime Minister David Cameron and French President Francois Hollande have agreed to back the 55-year-old Stoltenberg.
It said Stoltenberg's appointment could be announced as early as Monday when Obama is due to meet other leaders of the Group of Seven industrial democracies in The Hague. Obama will hold talks with current NATO Secretary-General Anders Fogh Rasmussen in Brussels on Wednesday.
However, the NATO diplomat said he did not think the announcement will be made yet as Rasmussen still has six months left in the post.
Rasmussen, a former Danish prime minister who took office in 2009, is due to step down at the end of September after a September 4-5 NATO summit in Wales which will mark almost the end of NATO's combat mission in Afghanistan.
An aide to Hollande said discussions were under way this week on candidates for the NATO post and NATO foreign ministers would also discuss it when they meet in Brussels next week.
If he gets the job, Stoltenberg will face a new challenge from a Russia which is flexing its military muscles. Russia's occupation of Ukraine's Crimea has put a chill on attempts by NATO and Russia to work together.
Newer NATO members in eastern Europe, including the ex-Soviet republics in the Baltics, are looking for reassurance from NATO that they would be covered by NATO's article 5 mutual defense clause if they were attacked.
Obama said in an interview with Dutch daily De Volkskrant that his message to allies during his European trip this week would be to provide that reassurance.
"Let nobody question the U.S. support for the safety of Europe. As NATO allies we have a holy duty based on Article 5, to defend one another. That will never change," Obama said.
NATO's Supreme Allied Commander Europe, U.S. Air Force General Philip Breedlove, said on Sunday that the Russian tactic of calling snap military exercises to ready forces for possible cross-border incursions should lead NATO to rethink the positioning and readiness of its forces in eastern Europe.
Many European NATO allies have slashed military spending in response to the financial crisis, creating military gaps that have alarmed the Americans, and Stoltenberg will have to continue the drive by Rasmussen to persuade them to reverse the spending cuts, or at least halt the slide.
Stoltenberg is considered a skilful economic operator who got Norway through the global financial crisis relatively unharmed as the government used its massive stored oil wealth to boost spending, create demand and keep unemployment low.
Stoltenberg's governments backed NATO's military campaigns in Afghanistan and Libya. During his time as prime minister, the country also began to place orders for Joint Strike Fighter aicraft from U.S. manufacturer Lockheed Martin Corp to replace an ageing fleet of F-16s.
Traditionally, NATO's top military commander is an American and its political leader is a European. NATO diplomats said others in the running to succeed Rasmussen were former Italian Foreign Minister Franco Frattini, Polish Foreign Minister Radoslaw Sikorski and Belgian Defence Minister Pieter de Crem.
(Additional reporting by Balazs Koranyi in Oslo, Fredrik Dahl and Justyna Pawlak in The Hague, Elizabeth Pineau and Paul Taylor in Paris, Anthony Deutsch and Geert de Clercq in Amsterdam, Jeff Mason in Washington; Editing by Angus MacSwan)