Luxembourg said Monday the Belgian authorities had failed to ease its concerns over the safety of at least one of Belgium's ageing nuclear reactors.

"We are not reassured because we did not get answers to all of our questions and so there is still a security gap," said environment ministry advisor Olaf Munichsdorfer.

"We are not totally convinced that all has been done to establish the origin of the cracks," he said, referring to fissures found in two Belgian reactors.

"We also said Luxembourg is exposed to this risk," added Munichsdorfer, who visited Belgium with Luxembourg Development Minister Camille Gira.

Gira earlier met Belgian Interior Minister Jan Jambon to discuss the presence of micro-cracks in the Tihange 2 reactor pressure vessel.

Tihange is about an hour's drive south of Brussels and the same distance from Luxembourg, which has become increasingly concerned, along with Germany, about the safety record of Belgium's nuclear plants.

The authorities found tiny pressure vessel cracks in 2012 in Tihange 2 and also in the Doel 3 reactor just north of Belgian's commercial hub and major port, Antwerp.

Closed down for repairs, they were both restarted late last year, with Doel 3 halted again after a water leak.

Both Electrabel, the national electric operator, and the Belgian nuclear watchdog AFCN say there is no danger at either Tihange or Doel despite the micro-cracks.

"We resumed service following an audit from a US research firm, an international firm that guaranteed the structural integrity of the vessels," Electrabel spokeswoman Florence Coppenolle told AFP earlier this month.

Munichsdorfer said Jambon has invited Luxembourg to send an official at an unspecified date to inspect the Tihange plant, just as a Dutch minister will tour the Doel plant on Wednesday.

A German official is also due to visit the Tihange plant, he added.

He said the two neighbours also agreed to "reinforce the exchange of information on the nuclear (issue)."

Jambon spokeswoman Anne-Laure Mouligneaux said the meeting with Gira took place in Jambon's offices with delegates from the AFCN as well as Belgian Energy Minister Marie-Christine Marghem.

"We listened to their questions and replied to them. We have explained the security measures," Mouligneaux told AFP, confirming that the two sides agreed on a visit to Tihange and to boost communication.

"We stressed that security, particularly nuclear security, is a priority," she said.