Greenpeace on Friday slammed "alarming gaps" in EU-wide safety checks on nuclear plants, notably for failing to address "the unthinkable" after Fukushima.

Seeking to ease public concerns following the March earthquake and tsunami that triggered Japan's nuclear crisis, the European Commission and national atomic operators struck a deal to launch stress tests on the European Union's 143 reactors in June.

But environmental watchdog Greenpeace said in a statement that early analysis of reports issued so far "reveals alarming gaps in results."

"Multiple-reactor failure that struck at Fukushima was supposed to be examined, but is missing in results. The threat of airplane crashes were also a promised part of tests, but are largely ignored."

In countries where it said national regulators were more independent from operators, such as in France, "tests were more thorough".

But it said Britain, the Czech Republic and Sweden "have failed to publish substantial information", comparing a seven-page Czech report on its six reactors to Slovenia's 177 pages on its single reactor.

In a response, the EU executive retorted that Greenpeace's assesment was based on provisional results that national nuclear operators had been asked to provide by August 15.

"It is by 31 October that nuclear operators have to complete all their investigations and send the final results of the stress tests to the regulators," EU energy commissioner Guenther Oettinger said in a statement.

"We are still in the process of doing the stress tests. By the end of the year, the regulatory authorities have to submit their final reports," he added.

"If these final reports contain deficiencies then I will not hesitate to intervene and ask for improvements."

Greenpeace nuclear policy advisor Jan Havercamp said reports seen by the group ignored town and city evacuation plans and failed to consider reactor age, as well as glossing over the risk of airplane crashes or multiple reactor collapse.

"Fukushima taught us to think the unthinkable," he said.