NATO leaders were keen to show unity Friday at NATO's 60th anniversary summit, but divisions linger over the future role of the military alliance and ties with Russia, officials and experts said.

The issues were touched on by the 28 NATO heads of state and government during an evening dinner.

But they will be treated in depth in the new "strategic concept" -- essentially a new mission statement -- that NATO headquarters in Brussels will be called upon to draw up, before the next summit in Lisbon late next year.

The idea is to update the old concept, sealed in 1999, by addressing modern threats, such as cyber attacks, terrorism, piracy, energy security and the fallout from climate change.

The first step at this summit will be to adopt a "Declaration on Alliance Security" -- a reflection paper drawn up as a first step in the process.

The text is only a page and a half long, but drafting it has again exposed old divisions and tensions at the world's biggest military alliance.

"There are deep differences," one NATO diplomat acknowleged, but declined to go into detail.

A senior military officer said: "It's no mystery. There are different sensitivities at NATO, notably about Europe and its defence capabilities, and above all Russia -- two themes touched on in the declaration."

Simon Koschut, an associate fellow at the German Council on Foreign Relations, said Russia's energy might, and particularly the dependence of European nations on its natural gas reserves, is cause for concern.

"For the Americans, there is the problem that Germany, that the Europeans, could be vulnerable to blackmail" from Moscow, he said.

But since Russia's conflict with Georgia last August, he said, its neighbours in Norway and the Baltic states "are also making plans based on the scenario of a threat to their security" in the classic sense of the term.

Norway in particular feels threatened by Russia's claims on the Arctic region, as the ice cap melts due to global warming and opens up new areas to the hunt for fossil fuels.

In a report Friday, Jonathan Eyal of the Royal United Services Institute in London said there was an "unavoidable clash" of interests between the West and Russia, because Moscow has "imperialist ambitions" and employs "Cold War logic".

Relations with Moscow, the reason NATO was formed 60 years ago, aside the old debate about the alliance's possible role as a global policeman is again stirring tensions, with the United States among those pushing hardest.

US National Security Advisor James Jones -- previously NATO's top military officer -- said the alliance "might have to be able to do things a little bit more rapidly in order to prevent conflict," rather than simply react to events and spend months deliberating before taking action.

But the senior NATO military officer conceded that "many amongst us do not want to play the role of the global policeman. But whether you like it or not, we are living in a globalised world."

"If there is a security problem, be it in the North or the South Pole, NATO should study it," he said. However he underlined: "NATO will only ever intervene if there is unanimity in its ranks."

But the question remains whether this will be enough to satisfy Germany and France, who don't want to see NATO's mission expanded too far beyond its original concept as the defender of the Euro-Atlantic area.