The $15 million dispute over the division of costs for the new Minnesota Vikings stadium will go at least several more weeks.

Three days of closed-door discussions have not yielded an agreement between the Minnesota Sports Facilities Authority (MSFA) and Mortenson Construction over who should pay for some of the work on U.S. Bank Stadium.

"Mediation is underway and will continue for at least the next month. All parties remain committed to the process and continue to be engaged," MSFA Chair Michele Kelm-Helgen said in a terse prepared statement. A Mortenson spokesman said only that the company would "concur" with Kelm-Helgen's statement.

That may be the public presentation of the talks, but the disagreement is far from routine. Mortenson historically settles disputes privately but took the rare step of filing for mediation in August after a year of closed-door talks with the MSFA had failed to close the gap. If mediation fails, the next step will be binding arbitration.

The underlying disagreement is over design changes to the project. Mortenson says it's owed money for costly changes that went beyond the original plans. The MSFA counters that nothing is owed because the changes should have been anticipated in Mortenson's original bid for the work

The contractor is trying to protect its investment and profit on the project. The MSFA is trying to do much the same with taxpayer money.

As of Thursday, an MSFA spokeswoman said the schedule for future mediation discussions had not been set.

The dispute has caused concern on the job site. Last summer, two subcontractors sent written pleas to the MSFA for a settlement, saying they were on the verge of bankruptcy. Mortenson then paid the subcontractors itself rather than wait out the mediation.

The $1.1 billion Vikings stadium is the largest public-private effort in state history. Team owners Mark and Zygi Wilf are paying about half the cost, with taxpayers paying $498 million.

The building is on track to open for the 2016 season. The roof is expected to be entirely in place by the end of November, so the workers will be indoors during the winter.