"If we ignore this cancer, it's not going to go away, it's going to kill all of us," Cindy Warmbier said.
Secretary of State Mike Pompeo refused to say if he believed North Korean leader Kim Jong Un didn't know about the mistreatment of American student Otto Warmbier.
The president spoke out after facing criticism over his comments on Warmbier.
Otto Warmbier was arrested in North Korea for "hostile acts aimed to subvert the country" and died days after his release.
Warmbier was sentenced to 15 years hard labor in North Korea, but he was released after 18 months.