A spree of denial-of-service (DOS) attacks against Web sites in Estonia appears to be subsiding, as the government calls for greater response mechanisms to cyber attacks within the European Union.

The attacks, which started around April 27, have crippled Web sites for Estonia's prime minister, banks, and less-trafficked sites run by small schools, said Hillar Aarelaid, chief security officer for Estonia's Computer Emergency Response Team (CERT), on Thursday. But most of the affected Web sites have been able to restore service.

"Yes, it's serious problem, but we are up and running," Aarelaid said.

Aarelaid said analysts have found postings on Web sites indicating Russian hackers may be involved in the attacks. However, analysis of the malicious traffic shows that computers from the U.S., Canada, Brazil, Vietnam and others have been used in the attacks, he said.

Experts from the North Atlantic Treaty Organization (NATO) are helping Estonia investigate the attacks, Aarelaid said.

Press reports also speculated that tension between the two countries may have resulted in a coordinated campaign by Russia against Estonia. Last month, Estonia irked Russia by moving a Soviet-era World War II memorial of a bronze soldier, sparking protests. Aarelaid dismissed the theory, saying Estonians were also divided on the issue.

A DOS attack involves commanding other computers to bombard a Web site with requests for data, causing the site to stop working. Hackers use botnets -- or groups of computers they've infected with malicious software -- to launch an attack.

It's difficult to trace who controls botnets, as the networks involve compromised computers located around the world.

"If you have an unknown number of attackers with different skills and capabilities, it's quite painful," Aarelaid said.

In Brussels on Monday, Estonia's defense minister, Jaak Aaviksoo, called for the development of a stronger capability to respond to cyber attacks within the European Union.

"Extensive cyber attacks against Estonia show clearly that this matter should be seriously dealt with and relevant information exchange with one another," Aaviksoo said.