The Czech Republic has been hit by an unprecedented wave of cyber attacks this week, with investigators stumped over their origin amid concerns they could lead to worse mayhem.

"We don't know anything about the motivation for now, because no one has claimed the attacks," Radek Holy, spokesman for the National Cyber Security Centre, told AFP following attacks on media, banks and telecommunications websites.

"The character of the attacks gives us absolutely no clues about the reasons" behind them, he added.

On Monday hackers launched a DDoS (distributed denial of service) attack on leading Czech news websites, blocking their servers with hundreds of thousands of requests.

They targeted, the most popular Czech search engine, on Tuesday before taking on the Prague Stock Exchange, the central bank and several commercial banks on Wednesday, and then two national mobile operators Thursday.

According to Karel Kucharik, head of the country's police cyber crime squad, this week's assault constituted "the biggest attack of its kind in the Czech Republic since the Internet was launched."

Hany Farghali, spokesman for targeted Telefonica Czech Republic, said the IP (Internet provider) addresses behind the attacks were Russian.

"I can confirm that we are cooperating on the international level," Czech police spokeswoman Marketa Johnova told AFP, without disclosing details "for tactical reasons."

But Holy was sceptical about the Russian link.

"We definitely can't say this attack has come from Russia. The IP addresses are not only Russian, they are from other parts of Europe too," he said, explaining the attackers were taking control of drone computers.

The computers' "owners may not even know about it," Holy added.

Hackers on Wednesday led an assault on the Czech central bank and major commercial banks including Ceska sporitelna owned by Austria's Erste Bank, Komercni banka owned by France's Societe Generale, CSOB, which is in the hands of Belgium's KBC, and the local branch of Austria's Raiffeisenbank.

Czech media were awash with speculation the following day.

They suggested it was work of a solitary hacker looking to boost his ego, or a protest movement similar to Anonymous, or a test ahead of a more serious assault.

Holy said this was "a possibility which is however impossible to confirm."

Mikulas Ferjencik, deputy head of the Czech Pirate Party, insisted he knew nothing about the whereabouts of the attacks.

"We still don't know who's behind it. It's a bit surprising that no one has raised any claims," he said.

"We have some people who are close to the hackers' community and we have no information yet," Ferjencik said, adding he was afraid the attacks might lead the government to tighten up a cyber security bill now in the works.

"I still think the culprit is from the Czech Republic," he told AFP.

Holy said that although he wasn't taking the attacks lightly, they were "not dangerous, rather unpleasant" for the targeted institutions.

"These attacks can't damage anything, they really merely burden the servers," he said.

"It's as if a crowd of people surrounded a building, blocked the door and the road and prevented people from going in or out, but no one would steal anything from the building itself," added Holy.

[Image via Agence France-Presse]