Major banks are stepping up their efforts to curtail losses from souring mortgages, with Citigroup Inc. becoming the latest institution to adopt initiatives aimed at helping at-risk borrowers remain in their homes. With defaults mounting, lenders including JPMorgan Chase & Co. and Bank of America Corp. have become more aggressive about modifications to mortgage agreements. The government is also working on an ambitious plan to help around 3 million borrowers avoid foreclosure, but details have yet to be released. More than 4 million American homeowners with a mortgage were at least one payment behind on their loans at the end of June, and 500,000 had started the foreclosure process, according to the most recent data from the Mortgage Bankers Association. Citigroup announced late Monday that it won't initiate a foreclosure or complete a foreclosure sale on any eligible borrower who seeks to stay in a home if it is the borrower's principal residence, the homeowner is working in good faith with Citi and has sufficient income to make affordable mortgage payments. Citi said it is also working to expand the program to include mortgages the bank services but does not own. Additionally, over the next six months, Citi plans to reach out to 500,000 homeowners who are not currently behind on their mortgage payments, but who are deemed as potentially needing assistance to keep current with their payments. This represents about one-third of all the mortgages that Citigroup owns, the bank said. Citi plans to devote a team of 600 salespeople to assist the targeted borrowers by adjusting their rates, reducing principal, or increasing the term of the loan.