Pro Rugby America, in a statement, described Pro12 as an "interloper" and claimed the league "does not have a legitimate business plan."
Pro12 officials held talks about US and Canadian sides being involved, which had been backed by World Rugby.
"It's ridiculous. What they're doing is just disrespectful," Pro Rugby America chief Doug Shoninger told BBC Wales.
"I understand Pro 12 is under a lot of stress, a lot of problem, but it's not our problem. And they need to solve that problem where they sit."
Pro12 is made up of four teams each from Wales and Ireland plus two each from Scotland and Italy.
Pro Rugby America is sanctioned by governing body USA Rugby to run a professional rugby division in the United States.
The US league started in April with five teams based in Denver, Ohio, Sacramento, San Diego and San Francisco and Shoninger told BBC Radio Wales they were negotiating a deal with the southern hemisphere Super Rugby league.
"We're bidding on a Super Rugby team," he said.
"We're talking to big established countries, ones I feel that have player exchange at academy levels.
"We're very honest about who we are - we have the potential great commercialism and we need the support of the rugby world to build up our infrastructure."
Pro12 is commercially smaller than the English and French professional leagues, and in August, Scottish Rugby chief Mark Dodson said they were "seriously investigating" the possibility of expanding into the American market.
The two Italian teams that compete in the league, Zebre and Treviso, have faced calls to be dropped from the Pro12 and were almost expelled in 2015 over money owed to the rest of the teams.
Pro12 Rugby declined to comment.
Plans for American sides to join the Pro12 have been dismissed by the company running the pro game in the US.