U.S. Democratic presidential candidate Hillary Clinton speaks during a campaign stop at Grand View University in Des Moines, Iowa January 29, 2016. REUTERS/Brian Snyder

WASHINGTON (Reuters) - Seven email chains from the private server Hillary Clinton used when she was U.S. secretary of state are being withheld from public release for containing “top secret” material, the Associated Press reported on Friday, without citing a source.

The U.S. State Department is scheduled to release another batch of emails from Clinton’s server on Friday. Clinton, who is seeking the Democratic presidential nomination, has been embroiled in a controversy about her use of a private server for government business.