WARSAW, Poland -- Russian and Polish officials confirmed Monday that they have carried out tit-for-tat diplomatic expulsions in an espionage affair that highlights intensified efforts by Moscow to penetrate NATO counties and a new determination by the West to fight back.

As tensions grow over Russia's military incursions in Ukraine, espionage also appears to be returning to Cold War levels. Russia and several NATO members have been accusing each other of stepped up spying, with diplomats allegedly playing key roles in the activity.

Russia's Foreign Ministry said that Polish authorities took the "unfriendly and unwarranted step" of expelling some of its diplomats -- and that Moscow retaliated by kicking out Polish diplomats.

"In connection with this, the Russian side has undertaken adequate response measures, and a number of Polish diplomats have 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 sent home 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 the right to work in the Czech Republic in 2006 on suspicion of spying.

Sweden's security service SAPO has said that Russia has increased its espionage activities in Sweden since the Ukraine crisis.

The Czech Republic also accuses Russia -- and China -- of increasing its espionage on its soil even before the crisis in Ukraine began, and said that both of those countries' embassies use intelligence officers under diplomatic cover.

"In 2013, the number of such officers at the Russian embassy was extremely high," the Czech counter-intelligence agency BIS said in its annual report published last month. It said spies also pose as tourists, academics and businessmen.

Also Monday, Germany said one of its diplomats in Moscow was expelled in a retaliatory measure. Though German authorities wouldn't say more, the magazine Der Spiegel, which first reported the case, said the diplomat was kicked out after a Russian diplomat was forced to leave Germany over accusations he spied there.