Ukraine's top security body said on Saturday that it and the national news agency had been hit by cyber attacks, the latest suffered by state organisations since the start of the crisis over Crimea.
The Ukrainian authorities said last week the country's telecommunications system had come under cyber attack, with equipment installed in Russian-controlled Crimea used to interfere with the mobile phones of members of parliament.
In the latest incident, unidentified sources launched denial of service (DoS) attacks which are intended to make a machine or network resource unavailable to its intended users.
"There was a massive DoS-attack on communication channels of the National Security and Defence Council of Ukraine, which was apparently aimed at hindering a response to the challenges faced by our state," the Security and Defence Council said.
It said the Ukrainian state-run news agency Ukrinform had suffered a similar attack.
The Ukrainian authorities said last week the country's telecommunications system had come under attack, with equipment installed in Russian-controlled Crimea used to interfere with the mobile phones of members of parliament.
They said that some Internet and telephone services were severed after Russian forces seized control of airfields and key installations in Ukraine's Crimea region.
The government has told ministries to take urgent measures to prevent their websites being blocked and ordered law enforcement bodies to investigate "evidence of cyber attacks and punish those responsible in their implementation".
The Ukrainian security chiefs did not say who was behind the communications disruptions.
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