More than 20 websites across four government ministries could not be accessed yesterday evening but were restored on the same day, Matsuno said.

The government has not identified any information leaks and was looking into whether the failure was caused by a denial-of-service (DDoS) attack, he added.

In a DDoS attack hackers attempt to flood a network with unusually high volumes of data traffic in order to paralyse it.

The pro-Russian group “Killnet” said on social media it was responsible for the attack, public broadcaster NHK reported.

“We are aware that the (Killnet) hacker group suggested it was behind an attack, but at the moment we are still investigating the cause of the failure, including its involvement,” Matsuno said at a daily news briefing.

“We understand that the (Killnet) hacker group has threatened several countries of cyberattacks, and that some say they are linked to the Russian government. Given our position as the government, we will not respond to that,” he added.

The attack temporarily blocked access to websites, including the Digital Agency’s e-Gov administrative portal, NHK said.