German government websites including Chancellor Angela Merkel's page were hacked on Wednesday in a cyber attack claimed by a group demanding that Berlin sever official ties with the Ukrainian government.

The attack, against which counter-measures were taken, had left the sites periodically inaccessible since 10 a.m. (0900 GMT), her spokesman Steffen Seibert said.

"Our service provider's data center 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.