Major League Soccer commissioner Don Garber says St. Louis and Sacramento are the leading candidates for the next round of expansion.
Garber said "it's fair to say they're front-runners" at a meeting with the Associated Press Sports Editors on Thursday.
He said other candidates, in order of priority, were Detroit, San Diego, San Antonio, Austin, and Cincinnati.
In a visit to Sacramento last week, Garber confirmed plans to expand the league to 28 teams.
MLS currently has 20 clubs, with Atlanta due to join in 2017 and a second Los Angeles team the following the year. Minneapolis-St. Paul also has been awarded a team and could open in 2017, and the league hopes David Beckham's tentative Miami team could start play in 2018.
That leaves four spots remaining for Garber's seven candidates.
Gaber said last week that "we hope and expect" that one of the teams in the next wave will be the USL's Sacramento Republic, which draw around 10,000 fans per game. The commissioner said the California capital could have an MLS team as early as 2020.
"Sacramento is MLS ready," Garber said on Thursday. "They've got 9,500 season tickets, good in any pro sports league. ... There's not a lot of competition there, and we seem to do better in those markets with less competition. There's an ownership group that's solid.''
Garber said he told the Republic "the last piece of the puzzle" was for the team to obtain commercial commitments and sponsorships.
In February, Garber outlined the same 2020 timeline for St. Louis, telling the St. Louis Post-Dispatch that "we believe it's MLS-ready." The Missouri city's prospects skyrocketed after the NFL's Rams announced in January they were leaving for Los Angeles.
"What held us back in St. Louis for 20 years is we never had a viable stadium,'' Garber said on Thursday. "Now we're talking to a group of investors and we're able to capitalize on the Rams have left. So there is a site where it's ready.
"St. Louis is a very, very high potential market, and one of the things that makes it more strategic for us is its proximity to Kansas City.''
The San Antonio Scorpions left the NASL after last season when the team and its stadium were sold to the owners of the NBA's Spurs, paving the way for a new club to join MLS.
A year ago, Garber had mentioned Indianapolis as a possible city for expansion, but it was not on his list on Thursday.
Cincinnati was a new name to come from Garber, possibly sparked by last Saturday's game between FC Cincinnati and Louisville City FC that drew a USL-record 20,000 fans at Nippert Stadium.
Garber said he received a call this week from Cincinnati FC owner Carl Lindner, whose team just started play in the USL this season.
"He said, 'When can we come into MLS?'" Garber recounted. "I said, 'You've got to wait a while. We've been at this a few years. Just two good weekends doesn't necessarily mean you're going to have a sustainable fan base.'"
Garber remains confident Beckham's group will finalize a stadium deal and said it is working to complete its investor group.
"If it doesn't happen, it would surprise us, if not shock us, that they weren't able to cross all the T's and dot the I's," he said.
Still, he added Miami and Los Angeles have "got to get busy fast" with stadium construction to meet deadlines. There were no significant updates on the search for stadium sites by New England and New York City.
Information from The Associated Press was used in this report.