Washington (CNN) Former Homeland Security Secretary Kirstjen Nielsen on Tuesday said she left office because "saying no" and refusing to do things that others in the administration wanted was "not going to be enough."

"There were a lot of things that, there were those in the administration who thought that we should do, and just as I spoke truth to power from the very beginning, it became clear that saying no, and refusing to do it myself was not going to be enough, so it was time for me to offer my resignation," she said at FORTUNE Most Powerful Women Summit.

Her comments came towards the end of an intense and brief interview with Amna Nawaz, national correspondent for "PBS NewsHour," which focused primarily on Nielsen's controversial role in the separation of families at the border.

Nielsen, who resigned in April, was pressed multiple times on whether she regretted signing a memo that enforced separating families.

"I don't regret enforcing the law, because I took an oath to do that," Nielsen said, adding that her decision was to "enforce the law, not to separate families."

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