"We have got to move toward a peace effort here, and we can't say we are going to do it some time in the future," Mattis said at a Washington event.
"We need to be doing this in the next 30 days."
The United Nations has labeled Yemen the world's worst humanitarian crisis. After three and a half years of war, 22 million Yemenis — or three-quarters of the population — depend on aid supplies.
One of the Arab world's poorest countries, Yemen has been embroiled in a devastating power struggle between the Saudi-backed government and the Houthis since late 2014.
The conflict intensified when Saudi Arabia and other allies started an air campaign in March 2015 to halt the Iran-backed group's advance towards Aden, the temporary seat of the government.