Russia and Greece were poised on Wednesday to expel two of each other's diplomats in a rare dispute that Greek media said had been prompted by the issue of Macedonia, which expects this week to be formally invited to join the NATO alliance.

Athens will expel the two Russian diplomats and ban entry to two others on suspicion that they tried to undermine an accord clinched between Greece and neighboring Macedonia last month, the Greek daily Kathimerini reported, citing diplomatic sources.

That deal - whereby Macedonia will become formally known as the Northern Republic of Macedonia - will unlock a formal invitation from NATO for the tiny ex-Yugoslav republic to join the bloc, a move strongly opposed by Moscow.

Moscow will respond to the expulsion of Russian diplomats from Greece by expelling two Greek diplomats, RIA news agency said, citing a member of the upper house of parliament, Andrei Klimov.

Greece and Russia share the Orthodox Christian religion and have traditionally had warm ties. Greece did not join most other NATO allies in March in withdrawing diplomats from Moscow over the poisoning of a former Russian spy in England. Britain blamed Russia for that incident. Moscow denied involvement.

Greece accused the Russian diplomats of activities inconsistent with their status, including illegal activities against national security, Kathimerini reported, adding that these had included attempts at bribery.

Moscow has long opposed the eastern expansion of NATO, regarding it as a direct threat to its own security. Many countries that were once in Moscow's orbit have joined NATO and the European Union since the fall of communism in 1989.

Macedonia expects to receive the invitation to join NATO at a two-day NATO summit, though it will only become a member if it adopts the new name agreed in principle with Greece.

Greece had long argued that Macedonia's name implies a territorial claim over its northern province of the same name, and had previously blocked its neighbor's attempts to join NATO. Macedonia will hold a referendum on the name deal.

Kathimerini said Greek authorities were investigating whether the Russian diplomats had meddled in Greece's internal affairs and quoted diplomatic sources as saying the suspicions were solely focused on those four individuals. He also said there was also growing unease in Athens over the increased influence of the Russian Orthodox Church at Mount Athos, an all-male monastic community in northern Greece.