The Netherlands has "serious" concerns about neighbouring Belgium's ageing nuclear reactors, the Dutch environment minister said Wednesday during a joint inspection of one plant with her Belgian counterpart.

Belgium has recently opened up its two nuclear plants to inspections by ministers from Germany, Luxembourg and the Netherlands after a series of problems including shutdowns, leaks and cracks.

"There are concerns because of the age of the nuclear plant and the incidents that have been occurring," Dutch environment minister Melanie Schultz van Haegen told a press conference after an inspection at the Doel plant near the Dutch border.

"The incidents have not caused any risk to the nuclear site. Now it's not a nuclear incident but maybe tomorrow it is. These worries are serious and this is why we are here, to give maximum transparency, to show them what it is really about," she said.

But she added that "what the experts are telling me gives me the certainty at the moment that the Doel plant is safe."

Belgian Interior Minister Jan Jambon told the same press conference that Belgium's seven reactors -- at Doel and at Tihange further south -- were all safe despite the recent problems.

"The guideline of the Belgian government is that we don't give one milimetre on security, not one milimetre," he said.

The concerns have largely emerged since the Belgian government's decision in December to extend the lives of 40-year-old reactors Doel 1 and Doel 2 until 2025 under a deal to preserve jobs and invest in the transition to cleaner energy.

It had originally shuttered Doel 1, the country's oldest reactor, in February 2015, but just three days after it was restarted in December it had to be closed due to a generator problem. It has now restarted a second time.

Tiny cracks discovered in 2012 in the reactor pressure vessels of Doel 3 and Tihange 2 also caused lengthy closures of those two reactors. They were both restarted at the end of last year, one having to close quickly again, for a few days, after a water leak.

And the Doel 4 reactor was also shut down urgently in August 2014 after a leak in the turbine hall, caused by tampering, gushed out 65,000 litres of oil lubricant. The investigation continues.