National and local philanthropic foundations have committed $330 million toward a deal to avoid cuts to Detroit retirees’ pensions and to save the Detroit Institute of Arts’ renowned collection, federal mediators involved in the city’s bankruptcy proceedings announced on Monday.

The plan was a first both in the foundation world, which has not been a source of money to shore up public-sector pensions in the past, and in municipal bankruptcy cases, experts said. It also offered the first indication of progress in the intense mediation with Detroit’s creditors to resolve the city’s financial crisis. Those talks have been proceeding under strict secrecy guidelines.

Nine foundations, many with ties to Michigan — including the Ford Foundation, the Kresge Foundation and the John S. and James L. Knight Foundation — have pledged to pool the $330 million, which would essentially relieve the city-owned Detroit Institute of Arts museum of its responsibility to sell some of its collection to help Detroit pay its $18 billion in debts. In particular, the foundation money would help reduce a portion of the city’s obligations to retirees, whose pensions are at risk of being reduced in the bankruptcy proceedings. By some estimates, the city’s pensions are underfunded by $3.5 billion.

As part of the plan, which negotiators have been working on quietly for more than two months, the museum would be transferred from city ownership to the control of a nonprofit, which would protect it from future municipal financial threats. The foundations would stipulate that Detroit must put the money into its pension system, said Alberto Ibargüen, president of the Knight Foundation.