JPMorgan Chase CEO Jamie Dimon said Thursday that markets will be gripped by panic as the U.S. approaches a possible default on its sovereign debt. An actual default would be "potentially catastrophic" for the country, Dimon told Bloomberg in a televised interview. Treasury Secretary Janet Yellen has said the idea that the country could default should be "unthinkable" and would lead to economic disaster. "If it gets to that panic point, people have to react, we've seen that before," Dimon said. But "it's a really bad idea, because panic becomes something that is not good," he added.