Reuters reports that the seven teams said they are in talks with other clubs about joining the new venture.
The USL released a statement attacking the breakaway group, saying they would oppose its registration with US football authorities and that "there is misrepresentation, interference with USL business operations and substantial debt among the membership of the parties applying for certification."
The breakaway teams are USL champions Montreal Impact, beaten finalists Vancouver Whitecaps, the Carolina Railhawks, Miami FC, Minnesota Thunder, former USL team the Atlanta Silverbacks, and a new team from St Louis.
The USL was founded in 1986 and has two professional men's divisions, backed up by a youth and women's structure. It plans to continue without the rebels in 2010.
