In the United States., fights between environmental nonprofits and state governments over hydraulic fracturing -- or fracking -- have spanned from Pennsylvania to California. And in Europe, Poland has become the latest fracking battlefield.

Polish environmentalists say they have been monitored and visited by secret police for campaigning against coal-fired power plants and unregulated shale gas exploration, according to reports from Euractiv. Earlier this month, dozens of environmental groups  sent a letterto Polish prime minister Donald Tusk, complaining of an "unprecedented attack" on civil society organizations.

"Some of the experts working for us were [also] visited by secret service agents, asking about their international relations with foreign organisations or movements," one unnamed person told EurActiv.

A leader of the green NGO Eko-Unia also said that his friends have received visits from secret police officials asking about his environmental activities.

The environmental groups singled out one recent quote in particular by the Minister of Treasury, Mikołaj Budzanowsk, as evidence of pressure against the country's NGOs.

In an interview with the Polish media this month, Budzanowsk said, "This kind of organisation should accept that there are limits to its activities. In my opinion they have passed their limit. If I were ClientEarth (…) I would reconsider doing any further work against [the Polish Energy Group]."

Poland relies on coal for 90 percent of its energy supply, and has blocked the European Union's low carbon road map three times. In its quest to be at the forefront of a European fracking boom, Poland has granted 111 shale exploration licenses to ExxonMobil, Chevron and other firms.

Since 2010 about 25 exploration wells have been drilled in the country’s shale.

Polish activists have disrupted meetings of energy corporations over the past year, including with a flash mob and "Frack You" banner last November at a meeting between Halliburton, Talisman Energy, Dow Chemical and the Polish government.

Still, government ministers say the environmental groups may not be representing the best interests of Poles.

"It is rather unclear whether such [environmental] organisations represent the interests of local communities or rather some unidentified interests, which are not always very clear or pure of intention," Michał Gołębiowski, the deputy director the Polish treasury ministry's department of strategic projects, told EurActiv.