The attack was under the observation of German security officials, who were working to gather information on the hackers' goals and identity, sources told dpa.
It emerged late Wednesday that Russia-linked hackers blamed for a string of high-profile cyber attacks had infiltrated the German government's secure computer network.
The hack appears to have originated from the APT28 cyber espionage group, also known as Fancy Bear, which many security experts believe has ties with the Russian government.
Patrick Sensburg, a member of the parliamentary committee that oversees German intelligence agencies, told broadcaster ZDF it would take time to analyse the incident that the German government said had been "isolated" and contained.
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He said it was premature to link the cyber attack - as German media reports have done - to APT28, although he said there was sufficient evidence that the group had links to a Russian spy agency.
"One has to carefully examine a software like this one that is extremely complex," Sensburg told the broadcaster.
He said there had been rumours about a possible breach of government networks, but his high-level committee had not been informed about the attack by government officials.
Anke Domscheit-Berg from The Left party told German television that it's "scandalous" she and other MPs had to find out about the attack through the media.
A panel will receive a closed-door briefing from the government later on Thursday. A separate panel on digital issues also called an extraordinary meeting to discuss the breach.
