WASHINGTON (Reuters) - U.S. asset managers pleaded on Tuesday for a delay to international rules on variation margin, the collateral posted for swaps, saying in a letter to international regulators that many will not be ready by the March 1 start date.

Two trade groups, the Securities Industry and Financial Markets Association and the Investment Adviser Association, said that asset managers have "massive" amounts of work to complete before the operative deadline and need six more months.

The U.S. rule, part of the Dodd-Frank reform law passed in 2010, is intended to require adequate collateral for covering positions in swaps that are not cleared by a third party. It sets limits on what can be used as variation, or daily, margin and who must post it.

Other countries have similar requirements on variation margin set to come on-line around the same time.

"The associations believe that relief is urgently required to protect asset managers’ clients given that implementation efforts will not cover investors’ needs," they wrote in a letter.

The letter was sent to six U.S. regulators: the Federal Reserve Board, Commodity Futures Trading Commission, Comptroller's office, Federal Housing Finance Agency, Farm Credit Administration and the Federal Deposit Insurance Corporation.

The letter was also sent to four European regulators including the European Commission, as well as the United Kingdom Financial Conduct Authority and the Japan Financial Services Agency, all of whom have similar requirements slated to start around the same time.

"Asset managers, as fiduciaries serving these investors, continue to have concerns about their ability to hedge, manage investment risks, implement investment strategies, and achieve best execution on behalf of their clients," they wrote.

U.S. regulators may take the request seriously. The CFTC attempted to bring a different set of margin requirements for uncleared swaps on-line this summer and stirred up derivatives markets around the world because other countries had delayed implementing their rules. The agency, which regulates derivatives and commodities, then had to push back the compliance date for a month.

(Reporting by Lisa Lambert; Editing by Cynthia Osterman)