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Ukraine's prime minister has blamed the Russian secret service for a hack attack against German government websites.

A pro-Russian group has already claimed responsibility but this is the first suggestion that it was backed by the Russian government.

Prime Minister Arseny Yatsenyuk made his remarks ahead of a meeting with German Chancellor Angela Merkel.

But one expert questioned what real evidence there was for the claims.

The attack targeted webpages for Mrs Merkel and the German parliament. Both sites were inaccessible from 10:00 Wednesday until the evening.

A group calling itself CyberBerkut claimed to be behind it. Berkut refers to Ukraine's now disbanded elite riot police force who were accused of beating, torturing and shooting demonstrators.

Ahead of his meeting with Mrs Merkel, Mr Yatsenyuk was asked by German TV reporters if pro-Russian hackers from Ukraine were responsible.

"I strongly recommend that the Russian secret service stop spending taxpayer money for cyber-attacks against the Bundestag and Chancellor Merkel's office," he replied.

Tit-for-tat cyber-attack accusations are becoming more and more common even though it is notoriously difficult to pinpoint where an attack originates.

"It is interesting that countries are blaming each other for cyber-attacks even though the information they put in the public domain often doesn't substantiate their claims," security expert Prof Alan Woodward told the ������̳.

He said that he would have thought that governments would be sure "beyond reasonable doubt" before making such serious accusations.

"The international community seems very quick to blame each other on the balance of probability which doesn't seem enough to me," he said.

This week, the FBI added more to its theory that North Korea was behind the Sony Pictures hack in November, reiterating that IP addresses linked to the attack were used exclusively by the secretive nation.

Prof Woodward questioned the FBI claims.

"None of these addresses were actually in North Korea. They were in Singapore, Taiwan and all over the place."

He thinks that cybercrime is increasingly becoming part of the political agenda.

"It is interesting that all the rhetoric seems to be coming from countries that have existing tensions. They are using specific cyber-attacks for political point-scoring."

German intelligence agencies have said that they face about 3,000 cyber-attacks each day. About five of these come from foreign intelligence agencies, the head of German domestic intelligence agency said recently.