By Andrew Gray

VILNIUS, Feb 8 (Reuters) - Sometimes strident, sometimes sympathetic, U.S. Defense Secretary Robert Gates is walking a fine line in trying to win more public support and troops from Europe for NATO's war in Afghanistan.

Pentagon officials say Gates is trying to provoke a public discussion in Europe about the mission. But some of his recent tough comments run the risk that European governments will be less likely to provide the aid he wants.

At a meeting with NATO colleagues in Lithuania this week and at a weekend security conference in Munich, one of Gates' tasks is to bridge a gap in perceptions between Washington and some European nations about the very nature of the war.

While European states prefer to bill the mission as a reconstruction or peacekeeping effort, other nations such as the United States and Britain stress they are in a tough counter-insurgency war against Taliban militants.

Before flying to Lithuania on Wednesday, Gates said NATO was in danger of becoming a two-tier alliance, split between countries willing to send troops to "fight and die to protect people's security and others who are not".

That comment seemed particularly aimed at countries such as Germany, which has confined its troops to the safer north of Afghanistan while others fight intense battles in the south.

Gates also said he would "nag" allies once again to provide more troops and resources for Afghanistan. But a NATO source said ministers asked Gates to tone down public criticism.

"He's been told by a number of people, and not just the most obvious ones like the Germans, to stop it," the source said.

The Pentagon chief adopted a softer tone with reporters in Vilnius on Thursday, declaring the 42,000-strong Afghan mission was not in trouble and NATO was not in crisis.

FINE LINE

"The secretary's clearly walking a fine line between wanting to encourage, prod or, as he said, nag our allies into seeing what more they can do to fulfil the outstanding military requirements and not alienating our friends in Europe," Pentagon press secretary Geoff Morrell said.

"While it's inherently difficult to walk that fine line, it's clearly provoking discussion and that is a good first step," Morrell told reporters travelling with Gates.

Since Gates wrote to allies last month asking them to do more in Afghanistan, some German politicians and commentators have suggested Berlin consider sending troops to the south.

After taking office in December 2006, Gates showed an ability to speak diplomatically and cultivate ties with allies. But some comments in recent months have reminded some European politicians of his combative predecessor, Donald Rumsfeld.

He will need to be at his most persuasive at the Munich Conference on Security Policy on Sunday, when he is expected to appeal directly to European publics, arguing it is in their interest to support the war to prevent terrorism at home.

Polls suggest up to two-thirds of Germans are against the mission in Afghanistan, where Germany has about 3,200 troops.

France has also been ambivalent, although its government has changed tone under pro-American President Nicolas Sarkozy.

Paris resisted NATO's first out-of-area operation at first and regarded U.S. demands for more European troops as an attempt by Washington to get its own forces out.

Gates' task may be all the more difficult as divisions over Afghanistan also exist within the Pentagon.

While commanders on the ground point to successes such as preventing a Taliban spring offensive last year, intelligence reports show a rise in insurgent attacks, U.S. Marine Corps commandant Gen. James Conway said last week.

"It is a bit confusing at this point because we as a department need to see it the same way and quite frankly, at this point in time, we just don't," Conway told reporters. (Editing by Catherine Evans)

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