"We are a bit worried, and I've heard rumours of great Roman tactics to remove the grand prix from Monza and bring it to Imola or even Rome," he told the Italian media.
"We are committed as a region and made a decision today to tell the Aci (Italian automobile club), who are making the deal, of our willingness to contribute the amount that is missing, on condition of entering into (race organiser) Sias," Maroni added.
It is believed the EUR 5 million pledge is in addition to a further EUR 7 million contributed by Lombardy over a ten-year period.
"There is a willingness on the part of the region to contribute with resources, because the grand prix must remain at Monza," said Maroni. "I think it would be a mistake for the sport if it did not."
Italian reports said Monza's negotiations must end by late June, when F1 supremo Ecclestone wants to complete the provisional race calendar for 2017.