Defense Minister Jaak Aaviksoo said about 1 million computers worldwide were used over more than two weeks to attack government and corporate Web sites in the Baltic country, which is engaged in a bitter dispute with Moscow over Estonia's removal of a Soviet-era war memorial from downtown Tallinn.
"We identified in the initial attacks IP numbers from the Russian governmental offices," Aaviksoo told The Associated Press by telephone, referring to traceable Internet addresses. "There is not sufficient evidence of a governmental role but it indicates a possibility.
"We don't have the conceptual space to properly name these attacks but we clearly feel it as a threat to national security," he told the AP.
The Russian government has denied it is involved. Spokesman Dmitry Peskov said the attackers must have used a fake Kremlin electronic address to tarnish Russian authorities.
The attacks, which subsided this week, have also concerned NATO, which sent a cyber expert to Estonia to investigate, an official with the military alliance said.
The attacks against Estonian Web sites started after the April 27 removal of a statue known as the Bronze Soldier, which commemorates Red Army troops killed fighting the Nazis, but which many Estonians consider a bitter reminder of five decades of Soviet occupation.
The statue was removed from a downtown square and placed in a cemetery outside Tallinn, infuriating Moscow and triggering deadly riots by Estonia's ethnic Russians, who said the move was an affront to the memory of the fallen soldiers.
Estonia, which broke away from the Soviet Union in 1991, is a NATO and European Union member. The country of 1.3 million has one of Europe's quickest-growing economies and has established itself as an Internet pioneer. In March, it became the first country worldwide to allow online voting in a parliamentary election.
The government said instructions in Russian had been spread on the Internet on how to jam Estonian sites with denial-of-service attacks, in which hackers overload a single network by directing massive traffic to the site. At least one suspect has been arrested in Estonia in connection with the attacks, Defense Ministry spokesman Madis Mikko said. The cyber controversy is expected to surface at an EU-Russia summit this week in Samara, southeast of Moscow. It's one of several matters that has soured the EU's relations with Russia. Some Estonian officials said the attacks raised questions about whether a coordinated sabotage of a nation's Web sites could amount to an act of war. "If let's say an airport or bank or state infrastructure is attacked by a missile it's clear war, but if the same result is done by computers ... then what do you call it?" Mikko asked. "Is it a state of war? Those questions must be addressed." NATO systems in Estonia have not been affected, the NATO official said. "Because it's an Estonian problem it's also a problem for us. We do take security very seriously," said the official, speaking on condition of anonymity because of the sensitivity of the issue. Linnar Viik, a member of Estonia's highest information technology advisory board, called the attacks "well-orchestrated, systematic efforts." "We see very clearly that this is not just hacker behavior," he said. "The attacks are very clearly focused."