Greece has said it cannot support Macedonia's bid to join Nato, because of an unresolved dispute over its Balkan neighbour's name.
Greek Foreign Minister Dora Bakoyannis told reporters in Brussels that Athens backed inviting Albania and Croatia but could not consent to asking Macedonia.
The decision threatens to derail Nato's plans to invite all three Balkan states at a summit in Bucharest in April.
Athens objects to its neighbour taking the name of a northern Greek region.
It says the name implies a territorial claim on the region, which Macedonia denies.
Ms Bakoyannis made the comments in a speech handed to reporters after a meeting of Nato foreign ministers.
"Unfortunately the policy followed by the government of the Former Yugoslav Republic of Macedonia in its relations with Greece, particularly as concerns its intransigent stance and its actions of an irredentist and nationalist logic, do not allow Greece to take the same positive stance as in the case of Croatia and Albania," she said.
However, she said she hoped there was still time to reach "a mutually acceptable" solution in time for next month's Nato summit.
A UN envoy is currently trying to find a compromise for both countries.