SYDNEY, March 8 (Reuters) - One of Australia's top government bureaucrats on Wednesday demanded Russia crack down on the large number of cyber criminals operating in the country, saying their actions posed a threat to national security. The comments come as Canberra reforms its cybersecurity policy following a raft of cyber attacks on some of the country's largest companies. "The greatest density of cyber criminals, particularly those with ransomware, are in Russia," Michael Pezzullo, Secretary of the Department of Home Affairs, told the AFR Business Summit in Sydney. The move follows a rise in cyber attacks since late last year with breaches reported by at least eight companies, including health insurer Medibank Private Ltd (MPL.AX) and telco Optus, owned by Singapore Telecommunications Ltd (STEL.SI). The United States and Britain sanctioned several Russians accused of cyber attacks last month, saying ransomware attacks have paralysed businesses, schools and hospitals.

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