The discussions are still ongoing and the figure could change as several scenarios are still under review, the person said.
The guarantees would cover the cost of winding down parts of Credit Suisse and potential litigation charges, two people told Reuters.
Talks to resolve the crisis of confidence in Credit Suisse are encountering significant obstacles, and 10,000 jobs may have to be cut if the two banks combine, one of the sources said.
Swiss regulators are racing to present a solution for Credit Suisse before markets reopen on Monday, but the complexities of combining two behemoths raises the prospect that talks will last well into Sunday, said the person, who asked to remain anonymous because of the sensitivity of the situation. Credit Suisse, UBS and the Swiss government declined to comment.
(Additional reporting by John Revill. Editing by Elisa Martinuzzi and Paritosh Bansal)
By Oliver Hirt and Stefania Spezzati