BRUSSELS -- Faced with a newly aggressive Russia, NATO has been mulling how to react, but it is ruling out one option: rapid expansion.

Four would-be members, including the former Soviet republic of Georgia, have been informed that admission to NATO isn't in the cards anytime soon.

For some, that means dashed hopes. Macedonia's foreign minister, Nikola Poposki, told The Associated Press in a statement it was a "step backward."

The bottom line: The North Atlantic Treaty Organization will welcome no new members when President Barack Obama and other leaders convene for a summit in Wales in early September.

Analysts say that NATO members are worried about granting, or being perceived as granting, security guarantees that could quickly be tested by Russia. That's particularly true of Georgia, which has been waiting since 2008 for the U.S.-led military alliance to make good on its promise of admission.

Before taking over Crimea from Ukraine, Russia invaded and occupied two regions of Georgia nearly six years ago -- and NATO is reluctant to take any action that might provoke a riposte from Moscow.

"The conflict over Ukraine has made it clear to them at NATO they have to be careful, both about security commitments and credibility," said Liana Fix, an associate fellow at the German Council on Foreign Relations. "If you give Georgia their membership action plan but don't defend them if something happens, what does it say about your credibility?"

NATO won't publicly hang up the "No Vacancy" sign. Anders Fogh Rasmussen, the alliance's secretary-general, proclaimed recently that "NATO's door remains open. And no third country has a veto over NATO enlargement."

But even before Crimea's annexation, some NATO countries were experiencing "enlargement exhaustion" and had become reluctant to increase the alliance's membership rolls, said Jorge Benitez, senior fellow for trans-Atlantic security at the Atlantic Council, a Washington-based think tank.

The small Balkan nation of Macedonia was also assured of a membership invitation by NATO leaders six years ago, but will have to wait. The deal-breaker is an unresolved conflict over the country's name, which duplicates that of a Greek region. Greece as a NATO member has an effective right of veto.

By the end of 2015, NATO foreign ministers will assess whether "the time is ripe" to invite Montenegro, another former Yugoslav republic, to join, Rasmussen said.

The fourth NATO aspirant, Bosnia-Herzegovina, has been unable to pass a key condition set by the alliance: transfer of 63 defense facilities from local authorities to the central government, NATO officials said.

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