The number of cyberattacks against companies and government offices has been increasing in Japan since March, ahead of a Group of Seven summit the country is set to host in May, sources familiar with the matter said Saturday.

Troubles caused by distributed denial-of-service (DDoS) attacks, which attempt to disrupt the online traffic of a server or network by overwhelming it with data, were mostly fixed quickly, the sources said.

"There is a possibility that (Japan) is being targeted as the host of the G7 summit, and we have warned relevant organizations to stay alert," an official at the National Center of Incident Readiness and Strategy for Cybersecurity said.

Tsunehiko Suzuki, a professor of information engineering at Chukyo University, said the attacks may have come as part of preparations for launching a full-scale attack.

The series of attacks began around March 14 and are still ongoing, he said.

The summit will be held in Hiroshima between May 19 and 21, bringing together the leaders of Britain, Canada, France, Germany, Italy, Japan and the United States, plus the European Union.

West Japan Railway was attacked between April 20 and 21, while Tokyo Electric Power Company Holdings was similarly affected on April 22, with their websites briefly becoming inaccessible.

Some local governments, including Osaka, Aichi and Kumamoto prefectures, also came under attack. In Nara, prefectural and municipal officials suffered a delay in email servers and could not access the internet following an attack Wednesday.

Some websites of central government offices, including one for the Cabinet Office's public relations, were also affected.

It is believed that the same hacker group carried out the attacks due to the similarity of methods employed in the incidents, the sources said.