(Reuters) - Italian automaker Fiat <FIA.MI> must pay at least $342 million for a 3.3 percent stake in automaker Chrysler Group LLC that is held by a union-related trust, the trust claimed in court documents filed on Monday.

The United Auto Workers healthcare trust said Fiat's offer of $139.7 million for the Chrysler stake was substantially below the fair market value, according to documents filed in Delaware's Court of Chancery.

Fiat sued the UAW trust in September for failing to sell the stake. As part of the 2009 agreement with the U.S. Treasury as Chrysler emerged from bankruptcy, Fiat is able to exercise call options to purchase portions of the stake held by the healthcare trust.

(Reporting by Tom Hals in Wilmington, Delaware; Editing by Kenneth Barry)