WASHINGTON (Reuters) - Lawmakers in the U.S. House and Senate agriculture committees are working on a short-term extension to the expired U.S. farm bill, and hope to vote on the extension by Monday, the final day of 2012, a Congressional source said on Friday.

The proposed extension to legislation that expired in September would be for "some months" but less than a year, the source said.

If an extension is passed the United States would avoid reverting to 1949 "permanent law" and with it a potential spike in the retail price of milk.

(Reporting by Charles Abbott, writing by Ros Krasny)