German Environment Minister Barbara Hendricks has asked Belgium to close two nuclear reactors temporarily because of concerns over safety.
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.
Belgium's authorities have rejected the German request, saying there is no need "from a nuclear safety point of view".
Ms Hendricks called for the reactors to be taken offline in response to a report by Germany's independent Reactor Safety Commission.
She 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."
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 Belgium's 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.
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.
But Belgium's Federal Agency for Nuclear Control (FANC) reacted to Ms Hendricks's request with surprise.
The agency said a fortnight ago it had met German experts who had not raised any new issues. It said it was still convinced there was "no need to shut down these units" and its conclusions had not changed.
Belgium's nuclear difficulties
- Belgium has two nuclear plants at Doel, near the port of Antwerp, and at Tihange near Liege
- They have a total of seven reactors, which produce around 60% of the country's energy needs
- One of Doel's four reactors, Doel 4, was hit by an unresolved case of sabotage
- Doel 3, was shut down for 21 months after the discovery of micro-cracks in the reactor's pressure vessels
- A few days after being restarted, Doel 3 was shut down again on 31 December 2015 after a water leak was found
- At Tihange, a fire started in the electricity supply system on 27 December
- Micro-cracks were also found in the pressure vessels of a Tihange reactor
- 2 February 2016