Recordings of upcoming testimony by an aide to former Secretary of State Hillary Clinton are to be kept under seal over concerns the video might be used against the Democratic presidential front-runner.

U.S. District Court Judge Emmet Sullivan ruled in advance of Friday's scheduled deposition by Clinton's former chief of staff, Cheryl Mills. She is among a half dozen current and former officials to be questioned about Clinton's use of a private email server when she was the nation's top diplomat.

Sullivan has approved the depositions as part of a lawsuit filed by the conservative legal advocacy group Judicial Watch. Lawyers for Mills raised concerns that video of her testimony might be used to "as part of a partisan attack." Mills supports the release of a written transcript.