"From a foreign intelligence perspective, that would represent opportunity," Admiral Michael Rogers testified before the Senate Intelligence Committee.
The server would be a "top priority for foreign intelligence services," Rogers agreed in response to a question from Republican Senator Tom Cotton, of Arkansas.
The controversy surrounding Clinton's use of her private email while serving as secretary has threatened to overtake her campaign for the 2016 Democratic presidential nomination. A new poll by the University of New Hampshire for CNN and WMUR showed Senator Bernie Sanders of Vermont ahead of Clinton with 46% of the vote to 30% for her.
Critics say Clinton's decision to go outside of State Department's standard email protocol created a national security risk. Those concerns have only been compounded by revelations that some of the information contained in her emails is now classified.
Cotton had asked Rogers to respond to a hypothetical cabinet member's request to use a private, non-governmental server for government business.
"My comment would be, 'You need to ensure you're complying with the applicable regulations and structure… for your department,'" Rogers replied, adding that he doesn't know the rules for every federal agency.
Cotton then asked if the communications of the president's senior-most advisers were often targets of foreign-intelligence agencies, to which Rogers responded "Yes."
"If an NSA employee came up to you and said, 'Hey, boss, we have reason to believe that Russian foreign minister Sergei Lavrov or Iranian foreign minister Javad Sarif is conducting official government business on a private server, how would you respond?" Cotton pressed.
Rogers paused before finally replying that the situation would represent an "opportunity."
Clinton's email server is currently in the hands of the FBI, which is searching the machine and reportedly has been able to recover at least some her personal emails.
