* EU nuclear watchdogs agree details of "stress tests"
* EU member states to decide strategy on terrorist risk
* Tests to start before June 1, finish by end April 2012
(Adds quote, Swiss phase-out, anti-nuclear meeting)
BRUSSELS, May 25 (Reuters) - European nuclear watchdogs have agreed details of new safety checks on the region's 143 reactors and said a group would be set up to deal with the risks of a nuclear crisis arising from a terrorist attack.
By June 1, regulators will have to start checking power plants' resilience to earthquakes and tsunamis to avert any crisis like that at Japan's stricken Fukushima plant, the European Commission said on Wednesday.
The tests, which follow two months of dispute, will also address the ability of reactors to withstand more common threats such as forest fires, transport accidents and the loss of electrical power supplies.
"We've come up with very comprehensive testing criteria," EU Energy Commissioner Guenther Oettinger told reporters.
"At the European-level, we'll be inspecting the inspectors," he said. "Human error has played a role in the Fukushima accident, so therefore we felt human error and human action had to be part of the stress test."
Officials say that in Europe the most significant threat to reactors comes from terrorism, but Oettinger said that was best handled by national security agencies.
"I respect that some member states say they don't want to show their cards -- that could even abet terrorism," Oettinger told German radio station Deutschlandfunk.
He now plans to prod Europe's neighbours, Russia, Ukraine and Switzerland, to follow suit. Europe's divisions over nuclear power have deepened since Fukushima, with Britain and France remaining steadfast supporters, Italy shelving plans to build new plants and Germany taking steps towards a phase-out.
The Swiss government agreed a gradual phase-out of nuclear energy on Wednesday. [ID:LDE74O1MF]
A fundamental shift in energy strategy is under way.
Germany's suspension of its oldest seven plants has already increased demand for coal, and analysts predict an increase in long-term European gas demand. [ID:nLDE74M0S7]
"The role of nuke power will suffer a setback," Paolo Scaroni, the chief executive of Italian energy giant Eni (ENI.MI), told reporters. "There are several reasons to think the gas market will tighten again."
Austria, a vocal opponent which banned new plants in 1974, said all its main demands had been met.
"This really was a tough fight," Austrian Environment Minister Nikolaus Berlakovich told Austrian radio. "I welcome that a nuclear safety system is being set up for the first time at the European level. The nuclear lobby resisted it, of course."
While the stress tests will have no legal teeth, they will be reviewed by other national regulators and the details will be made public. That means any plant that fails will come under ever more intense pressure from the anti-nuclear lobby.
"In case an upgrade is not technically or economically feasible, we believe reactors shall be shut down and decommissioned," the European Commission said in a statement.
"A government has to explain to its public why it has taken a decision, or failed to act."
This could put particular pressure on plants without containment structures for reactors or fuel pools, or those that face seismic threats.
That might put the spotlight on Britain's gas-cooled Magnox reactors, Russian-made units in Slovakia, Hungary and the Czech Republic, and old boiling-water reactors in Germany, Spain, Sweden and Finland.
Green group politician Rebecca Harms challenged governments to take the threat of terrorism seriously.
"The proposed working group... smacks of being a face-saving exercise, which will fail to actually test the ability of key sites in Europe to withstand an attack, like a plane crash -- which it is widely known they cannot," she said.
Austria convened a meeting in Vienna on Wednesday, which it said was attended by 11 anti-nuclear countries including Denmark and Ireland.
Environment minister Berlakovich said he would like to "expand the family" to include countries like Poland and Italy that were still undecided.
But he played down the speed of change: "Anyone who says Europe will be free of nuclear power at the push of a button is a charlatan."
(Additional reporting by Annika Breidthardt in Berlin and Michael Shields in Vienna; Editing by Michael Roddy)