BERLIN, March 1 (Xinhua) -- Germany's federal government has experienced a serious and sustained attack on its data networks, local media reported on Thursday.

According to the reports, the attacks could have begun as early as December 2016 and lasted until Wednesday. German security forces said they had been monitoring the activity throughout the period in order to obtain information about the objectives and identities of the hackers.

Ole Schroeder, the parliamentary state secretary at the interior ministry told RedaktionsNetzwerk Deutschland (RND) that the situation was under control by German security authorities.

"This was an exceptionally successful operation of the federal security agencies. We have managed to isolate and control a hacking attack on the network of the federal government through excellent cooperation," Schroeder said.

German security circles cited by media sought to reassure citizens that no large transfer of data had taken place. Investigators involved in the issue believe that the attacks were made for espionage purposes by the "Russian hacker group APT28".

Nevertheless, members of the parliamentary committee which supervises national intelligence agencies in Germany were critical on Thursday that they had been informed too late about the incident.

Konstantin von Notz, vice-president of the parliamentary committee for the control of intelligence services (PKGr), complained that it was "extremely bad and disappointing" that news of the cyber attack had been circulated in media before reaching PKGr.

Similarly, Andre Hahn of the Linke party told press it would be a "clear breach of law" if the federal government in Berlin had known about the attacks without informing the parliamentary committee.

However, Stephan Mayer, the interior spokesperson for the Christian Democratic Union/Christian Social Union parliamentary faction, warned Germans not to panic over the attacks.

"We will not contribute to a serious investigation if we lose ourselves in conjectures and hypotheses," Mayer argued. He defended the decision of security authorities not to immediately inform the general public in order to be able to gather more evidence.

Citing an expert whose IT security firm had observed "targeted attacks on foreign and defense ministries in the European Union" over the course of several months, the newspaper Die Welt reported on Thursday that the hacks could have been part of a wider attack on the European Union (EU).