He said investigators found that Russian groups are meddling in the European election to push forward pro-Russia candidates and thus weaken European support for Ukraine against Russia's two-year-old invasion.
"The objective is to help elect more pro-Russian candidates to the European Parliament and reinforce a certain pro-Russian narrative in that institution," De Croo told reporters.
"Weakened European support for Ukraine serves Russia on the battlefield," he said.
De Croo said the Belgian investigation was launched after Czech authorities found pro-Russian agents active in Brussels seeking to influence, and even pay, European lawmakers to promote a pro-Russian agenda.
It appeared that cash payments were not made in Belgium though interference had occurred there, he said.
Western nations have repeatedly accused Russian operatives of using social media and the internet to spread false or misleading information to undermine them, promote Russia or attempt to sway public opinion election campaigns.
Russian authorities have repeatedly denied the accusations.
Ahead of the European elections in June, parties that have criticised EU support for Ukraine, such as France's far-right Rassemblement National, Austria's Freedom Party and Germany's AfD, are set to gain more votes than five years ago.
De Croo has said he had asked for an urgent meeting of the European Union's Agency for Criminal and Justice Cooperation (EuroJust) to discuss this matter, and suggested that OLAF, the EU anti-fraud office, should prosecute the case.
"We have a responsibility and our responsibility is to uphold that every citizen's right to a free and safe vote can be maintained," he said.
(Reporting by Inti Landauro; editing by Mark Heinrich)
