German Environment Minister Barbara Hendricks has asked Belgium to close two nuclear reactors temporarily because of concerns over safety, BBC News reports.
The two ageing Doel 3 and Tihange 2 reactors were taken offline in 2012 when defects were found in the walls of the reactors' pressure vessels.
They were restarted late last year amid concern in the Netherlands and Germany. Hendricks has said they should be taken offline until further inquiries are carried out.
Responding to a report by Germany's independent Reactor Safety Commission, the environment minister said that "on the one hand [the report] says there are no concrete indications that the reactor pressure vessels will not resist the strain; but on the other hand they say you cannot, according to today's knowledge, be sure that they'll resist every possible strain. And that's why we need further investigation."
Dutch politicians have also expressed concerns about the safety of the Doel plant, which was opened close to the border in the mid-1970s and has four ageing reactors. Its oldest reactor was briefly taken offline last week.
Germany aims to turn off all its nuclear reactors by 2022 and there are particular worries in the south-west of the country about the safety of the Tihange plant, some 60km (38 miles) from the German border. Two states, North Rhine-Westphalia and Rhineland Palatinate, have said they will take legal action against Belgian plans to extend the life of the two reactors until 2025.
German nuclear safety experts visited Belgium's Federal Agency for Nuclear Control last week, and the agency said they agreed on further co-operation.