U.S. Treasury Secretary Janet Yellen and Chinese Premier Li Qiang sounded a hopeful tone on U.S.-China cooperation on Sunday, though the American policymaker noted both sides cannot shy away from tough conversations.

The U.S.-China relationship can only move forward with direct and open communication, Yellen told Li, after arriving in the Chinese capital from the southern city of Guangzhou.

Li, in welcoming Yellen, said "China sincerely hopes that the two countries will be partners, not adversaries."