The FTC is tasked with protecting consumers.
Business groups are allied against the changes, arguing they will have to spend millions of dollars buying up new domain names to protect their brands.
"We're very concerned that the costs will be significant to consumers and businesses," Leibowtiz said.
Brad White, a spokesman for ICANN, said the plan already includes safeguards to prevent groups from registering fake domains.
"We appreciate the concerns raised by Chairman Leibowitz," White said. "ICANN has already committed to vigilantly monitor for abuses to the new [generic top-level domain] program. If and when any program abuses arise, there are mechanisms in place to address them."
But Leibowitz said he does not believe ICANN's program will be able to prevent scammers from ripping off consumers
"We don't believe the protections are adequate. We think there's too much uncertainty, but we will certainly be willing and expect sit down with ICANN and hear them out," he said.
On Thursday, the Senate Commerce, Science and Transportation Committee will hold a hearing on Thursday to examine ICANN's plan.