already left our country because of activities incompatible with their status," the Russian Foreign Ministry said, using diplomatic jargon for spying.

Poland's Foreign Minister Grzegorz Schetyna confirmed the tit-for-tat nature of the expulsions. "This is something like a symmetrical answer," he said.

The authorities gave no other details, but a journalist with Polish broadcaster TVN, Andrzej Zaucha, reported from Moscow that four Polish diplomats were told Friday afternoon that they had 48 hours to leave Russia, meaning they had presumably returned to Poland by Sunday evening.

Edward Lucas, author of "Deception: the Inside Story of East-West Espionage Today," said that NATO and European Union members have been worried about Russian spying for years but are only recently doing something about it.

"In the past, Russian 'diplomats' who were caught spying were senthome quietly with no fuss. The people they had cultivated or recruited were mostly let off with a warning, and perhaps shunted to other jobs," Lucas said. "Now NATO and EU countries are fed up. They are publicly expelling Russian intelligence officers who work under diplomatic cover. They are arresting and prosecuting those who work illegally. ... And they are also arresting and prosecuting the people they recruit."

Poland's expulsion of Russian diplomats comes a month after Polish authorities arrested a Polish military officer and a Polish-Russian lawyer on suspicion of spying. They remain under arrest as their cases are investigated. Separately, a Russian journalist was stripped of his right to work in Poland. The journalist, Leonid Sviridov, who works for the Rossiya Segodnya news agency, was also denied