"Our service provider's data centre is under a severe attack that has apparently been caused by a variety of external systems," he told a news conference when asked if Ukrainian hackers were responsible.
In a statement on its website, a group calling itself CyberBerkut took responsibility.
'Berkut' is a reference to the riot squads used by the government of Ukraine's former pro-Russian President Viktor Yanukovich, who was ousted during violent protests last February.
"CyberBerkut has blocked German Chancellor and the Bundestag's (lower parliamentary house) websites," it said on www.cyber-berkut.org.
The claim could not be independently verified.
"We appeal (to) all people and (the) government of Germany to stop financial and political support of criminal regime in Kiev, which unleashed a bloody civil war," it said.
Ukrainian Prime Minister Arseny Yatseniuk is due to meet with German President Joachim Gauck in Berlin later on Wednesday.
(Reporting by Michelle Martin; editing by John Stonestreet)
