Inside the company, everyone is totally focused on this deal, the source told Reuters on condition of anonymity.
Fiat and Chrysler face an April 30 deadline set by the U.S. government to present a restructuring plan for the struggling U.S. car maker, known for brands like Dodge and Jeep.
If they fail, the government has threatened to withhold extra funding for Chrysler and let it go bankrupt.
Fiat Chief Executive Sergio Marchionne made yet another trip to the United States early this week ahead of the publication of his group's quarterly results on Thursday, when he is expected to give an update on the talks.
Italian Prime Minister Silvio Berlusconi threw his weight behind Fiat's efforts to forge a deal with Chrysler in order to return to the huge U.S. market where its brand has been absent for more than two decades.
I don't know what the odds are, but I hope that this operation goes ahead and gives Fiat a big boost and the possibility to count for something in the world and the American market, he told reporters after a party meeting on Tuesday.
Those holding out for better terms are Chrysler's bondholders and union workers in Canada, who refuse to bear some of the costs that a partnership would entail.
Marchionne has threatened to give it all up, and Fiat Chairman Luca Cordero di Montezemolo said again on Tuesday that the group did not exclude looking for an alternative to Chrysler.
We look with interest at everything, we don't exclude anything, he was quoted by local news agency ANSA as telling reporters at an event in Milan. We look at everything that is worthwhile.
However, the source told Reuters that Fiat's staff were focused on Chrysler and making it their top priority.
The prospects of a deal have boosted Fiat's share price in recent sessions. It closed down 3.32 percent at 7.28 euros on Tuesday as the DJ Stoxx index of auto companies <.SXAP> was flat.
News reports have fueled speculation about the likely candidates for a Plan B, including General Motors
Under the terms of the proposed partnership with Chrysler, Fiat would get access to the U.S. market and a minority stake in Chrysler in exchange for technology to make small cars and access to overseas markets. No cash would change hands.
(Additional reporting by Francesca Piscioneri in Rome; Editing by Andrew Macdonald)