STRESA, Italy, May 25 (Reuters) - U.S. Treasury Secretary Janet Yellen said on Saturday that a loan for Ukraine backed by the income from frozen Russian sovereign assets is the "main option" for G7 leaders to consider in June but added that she doesn't want to "take anything off the table as a future possibility."

Yellen told reporters at the end of a G7 finance meeting in northern Italy that the plan has broad-based support and there is "quite a bit of work to be done" to make it a reality, adding that the 27-member European Union needs to endorse it.

"It needs to be fleshed out within the EU so that it can become a proposal that the EU endorses, and that's a lot of countries," Yellen said. "It's not a given, so I'm not saying this is a totally done deal." (Reporting by David Lawder Editing by Keith Weir)