The U.K.'s top cybersecurity official has confirmed for the first time that Russian hackers targeted British media, telecommunications and energy firms over the past year.

Ciaran Martin, head of the National Cyber Security Center (NCSC), made the remarks during the Times Tech Summit on Wednesday. His comments are one of the strongest acknowledgments so far from a top official of the extent to which the British government has faced cyberattacks from Russia.

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“That is clearly a cause for concern — Russia is seeking to undermine the international system,” Martin said.

The NCSC chief’s remarks follow a Monday speech by British Prime Minister Theresa May, in which she called out the Russian government directly for its cyberattacks.

“We know what you are doing,” May said. “And you will not succeed.”

She attributed Russian cyber warfare to the U.K.’s rationale for increasing its military and economic support of Ukraine. Her comments are her most direct condemnation to date of Russia’s cyberattacks against the U.K.

The country has been forced to confront the dangers of cyberattacks more directly after it fell victim to the North Korean WannaCry hacks earlier this year, which shut down hospitals across the country.

The U.S. is facing its own cyber threats from Russia after the country hacked the Democratic Party during the 2016 presidential campaign and sought to interfere in American politics around the same time, using social media and digital ad platforms.