By Emily Chasan

NEW YORK, May 18 (Reuters) - The Financial Accounting Standards Board, which sets U.S. accounting rules, voted on Monday to adopt two rules that could force financial institutions to bring more off balance sheet assets onto their books.

The rules will take effect at the beginning of 2010 and the FASB expects to issue final versions next month.

The changes, which affect two rules known as FAS 140 and FIN46(R), have been watched closely by investors amid concerns it could force financial firms to book trillions of dollars worth of troubled assets and raise more capital to offset risks.

The FASB has been working on the changes to securitization accounting for months, as it tried to improve disclosures relating to mortgage-backed securities, structured investment vehicles and other passive investments designed to be legally isolated from the banks that create them.

Those assets had traditionally been kept off bank books by using an accounting concept called the "qualified special purpose entity," or QSPE. But FASB Chairman Robert Herz has said repeatedly over the last year that those rules were "stretched" and "abused" in the years leading up to the credit crisis and needed to be changed.

The changes eliminate the QSPE concept that was used to keep assets such as mortgage-backed securities off of books.

Companies will also have to alter the ways they evaluate transfers of financial assets, perform more analysis about whether they have a "controlling interest" in special-purpose entities and increase disclosures to investors, under the new rules. (Reporting by Emily Chasan; Editing by Andre Grenon)