"Law enforcement authorities in a number of countries have arrested a number of individuals accused of corrupt activities related to NATO contracts," NATO spokeswoman Allison Hart said.

"NATO -- including the NSPA (NATO Support and Procurement Agency) -- is working closely with law enforcement to ensure that perpetrators are brought to justice."

Prosecutors in Belgium said one person had been formally arrested in the country after anti-corruption officers carried out searches.

"The investigation concerns possible irregularities in awarding contracts to defence contractors for the purchase of military equipment for NATO such as ammunition and drones," the Belgian prosecutors said.

"It would be more specifically about the possible passing on of confidential information by employees of NATO Support & Procurement Agency based in the Grand Duchy of Luxembourg, to defence contractors eligible to win these contracts."

The Belgian authorities said several former employees of NATO's procurement agency "are said to be involved".

Prosecutors in Luxembourg meanwhile said police in the country had carried out "various searches" and seized documents related to the case.

"Belgian, Dutch, Italian, Spanish and American authorities conducted simultaneous operations, leading to the arrest of suspects," the Luxembourg authorities said.