China controls some elements of the national security supply chain, the top U.S. cyber chief told Congress at a hearing Tuesday in remarks that hinted at the difficulty of walling off the Pentagon from Beijing’s influence.

U.S. Cyber Command chief and National Security Agency director Gen. Paul Nakasone warned against depending on Chinese-origin equipment even as he highlighted the impossibility of divesting entirely from Chinese supply chains in response to a line of questioning from Republican Florida Sen. Rick Scott. The cyber domain presents a vast field of targets and opportunities for China, including through trade in goods and commodities, Nakasone explained.