The attack on Wednesday took place before Prime Minister Arseny Yatseniuk was due to meet with German Chancellor Angela Merkel.
The Russian group demanded Berlin end support for the Ukrainian government.
"I strongly recommend that the Russian secret services stop spending taxpayer money for cyberattacks against the Bundestag and Chancellor Merkel's office," Prime Minister Arseny Yatseniuk told ZDF TV when asked if pro-Russian hackers from Ukraine were responsible.
The attack included web pages for Merkel and for the German parliament.
Merkel's spokesman, Steffen Seibbert, said counter-measures were taken but failed to halt the attack, which left the sites inaccessible from 10 a.m. Wednesday until the evening.
In a statement on its website, a group calling itself CyberBerkut claimed responsibility.
"Berkut" refers to the riot squads used by the government of Ukraine's former president, the pro-Russian President Viktor Yanukovich, who was ousted after violent protests last February.
The claim could not be verified.
It was believed to be the first successful prolonged attack on German government websites, which intelligence agencies say face about 3,000 such assaults daily.
About five of the daily attacks come from foreign intelligence agencies, the head of the BfV domestic intelligence agency said recently.
The attack comes after U.S. investigators said they believed North Korea had probably hired hackers for a massive cyber attack against Sony Pictures.
North Korea blamed the United States for internet outages it suffered soon after.
Source: Google News