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(Reuters) - At least a half-dozen major U.S. companies whose computers have been infiltrated by cyber criminals or international spies have not admitted to the incidents despite new guidance from securities regulators urging such disclosures.

Top U.S. cybersecurity officials believe corporate hacking is widespread, and the Securities and Exchange Commission issued a lengthy "guidance" document on October 13 outlining how and when publicly traded companies should report hacking incidents and cybersecurity risk.

But with one full quarter having elapsed since the SEC request, some major companies that are known to have had significant digital security breaches have said nothing about the incidents in their regulatory filings.