Romania has charged six men with treason for plotting a coup with links to Russia, prosecutors in the NATO ally said, accusing them of conspiring "to undermine the sovereignty and independence of the Romanian state".

In an unprecedented move, authorities in Bucharest arrested six citizens Wednesday for organising a paramilitary group with Moscow's support, linked to the expulsion of the Russian military attache to Romania and his deputy.

Romanian prosecutors detained six people on charges of forming an organised criminal group and treason, they said on Thursday, adding that members of the group repeatedly contacted agents of a foreign power both in Romania and in Russia.

Romanian prosecutors detained six people suspected of allegedly plotting to overthrow the government and withdraw the Black Sea nation from NATO with Russia’s help.

Romanian prosecutors said Thursday that they arrested and charged six people with treason, accusing them of colluding with Russia to "undermine" the country. The case has already led to the expulsion of two Russian diplomats — Russia's military attache and his deputy — who were declared persona non gratae on Wednesday.