The Belgian interior ministry has found itself the target of "sophisticated" cyber espionage, a spokesman told RTBF public television on Tuesday.
The Federal Public Service Interior's communications director, Olivier Maerens, insisted, however, that the ministry's servers were well protected and that the hackers had not succeeded in getting hold of the most sensitive data.
Federal prosecutors had launched an investigation to identify the origin of the operation, which data had been hacked and whether a foreign state was involved.
According to Belgian experts, the attack, which was uncovered in March, had been launched in 2019.
Another large-scale attack at the beginning of May had caused the Belnet network - which connects higher education establishments, universities, research centres and public administration - to crash.
But the aim of the attack against the interior ministry was not to jam the website or demand any ransom.
It was "more complex and well targeted, leading us to think it was espionage," according to the experts cited by RTBF.
Maerens said that "urgent action was taken to prevent the attacker's access" and server security had been strengthened.
Discovery of the attack had been kept secret so as not to expose the vulnerability of the system until it was protected.
EU leaders meeting in Brussels on Monday and Tuesday discussed the threat of cyber attacks during meetings on the bloc's tensions with Russia, which has been accused of launching a number of such operations.
"The level of Russian interference both with spies and with web manipulation has become truly alarming," Italian Prime Minister Mario Draghi told a news conference.
"We have to boost our protection, notably in terms of cyber security. We have to do everything, both at a national and EU level," Draghi said.