National Security Administration (NSA) director Mike Rogers on Thursday admitted in a Senate Intelligence Committee hearing that Democratic presidential candidate Hillary Clinton's private email server was a severe security risk.

Clinton is facing criticism for having used a personal email address during her entire four years as secretary of state, despite the fact that US President Barack Obama signed in a law saying government email accounts must be used on official business. Her email server is said to be recoverable, meaning she may face consequences for having sent classified information.

Senator Tom Cotton (R-AK) asked Rogers during the hearing on Thursday whether "the communications of the senior-most advisers to the president of the United States - even those that may be unclassified - a top priority for foreign intelligence services, in your opinion?"

"Yes," replied Rogers.

Cotton continued, asking, "if an NSA employee came to you and said, ‘hey boss, we have reason to believe that Russian foreign minister Sergei Lavrov or Iranian foreign minister Javad Zarif is conducting official business on a private server,’ how would you respond?"

"From a foreign intelligence perspective, that represents opportunity,” Rogers said, indicating the lapse would be capitalized on by tapping into the emails.

In an apparent attempt at damage control, Democratic Senator Dianne Feinstein of California tried to belittle Cotton's questions.

“I don’t know the relevance of that to this committee, although that’s just my opinion,” Feinstein said.

Rogers' assessment of the security risk posed by Clinton's private server echoes statements by former Defense Intelligence Agency director Michael Flynn, who in April said it was "likely" spies breached into Clinton's emails.