Germany has asked Belgium to take two nuclear reactors temporarily off the grid while questions about their safety are cleared up, an unusual diplomatic move that underscores German concerns about the plants.

Production at Belgium’s Tihange 1 nuclear reactor was halted for about 10 days in December because of a fire. Staffing has also been reduced to minimize the risk of unauthorized personnel gaining access to the plants after the November attacks on Paris and the March attacks on Brussels.

Environment Minister Barbara Hendricks said on Wednesday that the decision to request another shut down of the Tihange 2 and Doel 3 reactors came after Germany’s independent Reactor Safety Commission advised that it could not confirm the reactors would be safe in the event of a fault.

Deputy Environment Minister Jochen Flasbarth telephoned the Belgian Interior Minister on Hendrick’s behalf on Tuesday to request a shutdown pending further safety investigations. Officials did not specify a timeframe.

The core tanks at the 33-year-old Doel 3 and Tihange 2 reactors were built by Dutch company Rotterdamsche Droogdok Maatschappij, which has also built reactors in other countries.

The two reactors, both with about a gigawatt of capacity, were closed in 2012 and again in 2014 after a brief restart, after inspections unveiled tiny cracks in their core tanks.

But the Belgian regulator authorized a restart in November 2015 after finding that the cracks were hydrogen flakes trapped in the walls of the reactor tank and had no unacceptable impact on the plant’s safety.

“I consider it right that the plants are temporarily taken offline at least until further investigations have been completed. I have asked the Belgian government to take this step,” Hendricks said in a statement.

She added the move would send a strong signal to reassure Germany and show that Belgium is taking the concerns of its neighbors seriously.

Belgian nuclear regulator FANC expressed surprise at the German minister’s remarks, saying in a statement that it had explained the issue with the reactors at a meeting of international experts.

“The nuclear reactors at Doel 3 and Tihange 2 fulfill the highest security standards,” the agency added.

Spurred by the disaster at Japan’s Fukushima plant in 2011, Germany pledged to abandon nuclear power generation completely by 2022 in favor of other power sources.

Hendricks’ comments are the highest profile criticism of the Belgian nuclear reactors so far in Germany, with the region around Aachen and the state of North Rhine-Westphalia having previously voiced concern.

Last week, the state of North Rhine-Westphalia said it would join a lawsuit brought by the Aachen city region against the Tihange 2 reactor, which is roughly 65 kilometers (about 40 miles) away from the west German city.

Germany has long been nervous about the safety of the reactors and a working group of officials met earlier this month to discuss the issue. Flasbarth told reporters talks with Belgian authorities had been constructive.

He added the decision to make the request had not been taken lightly and that Germany would give the Belgian government time to respond.