European stress tests on nuclear power plants are still preliminary but have the potential to evolve into clear, objective standards valid for years to come, Energy Commissioner Guenther Oettinger said.

Green groups and politicians have strongly criticised the EU's response to Japan's Fukushima nuclear disaster earlier this year, saying they have not assessed multiple reactor failure or the threat of crashes by large aircraft.

Oettinger told Reuters the EU stress tests included floods, earthquakes, human or technical error and loss of power supplies, as well as plane crashes.

He also said they were still at an early stage.

I can answer to the Greens, be relaxed, wait and see, wait for Christmas and have a happy new year, said Oettinger.

A progress report to be delivered on Thursday will be tentative, with final contributions from member states expected by the end of the year under a first voluntary reporting phase.

After that, the EU will begin work on a peer review, drawing together all the findings.

The achievement so far, Oettinger said, was that all EU nations with operating nuclear plants were involved.

The first success maybe is all 14 member states who have operating plants are taking part. Some neighbouring states are taking part, he said in an interview.

At the moment, my ambition is to have the first objective and successful stress test. If we come to clear conclusions, then maybe this stress test is enough for the next 5-to-10 years.

Environmental groups have said the EU tests are so lax, all nuclear plants would pass them.

The EU stress tests will completely fail to assess risks in core areas that would be essential in terms of preventing nuclear accidents, Green Group EU lawmaker Rebecca Harms said in a statement.

Officials outside the Commission, as well as within it have said the testing process has exposed serious issues.

The head of the OECD's Nuclear Energy Agency Nuclear Development Decision said nuclear reactors older than 40 years might have to shut down and the head of the French nuclear safety agency said France needed to improve protection of safety mechanisms.

(Reporting by Barbara Lewis and Ilona Wissenbach; additional reporting by Christopher Le Coq)