(Bloomberg News) Real estate investment trusts that buy U.S. mortgage debt tumbled to the steepest losses since December 2008, on concern that their main source of financing will be roiled by European bank woes.

Mortgage REITs including Annaly Capital Management Inc. and American Capital Agency Corp. dropped as much as 6 percent today, according to a Bloomberg index tracking 33 shares. Losses over the past two days reached as much as 11.1 percent, the biggest fall in almost three years. The shares pared today's declines to 2.6 percent at 1:50 p.m. in New York.

France and Belgium pledged today to support Dexia SA after the bank's board met to discuss a possible break-up as Europe's sovereign-debt crisis reduced its ability to obtain funding. While the repurchase-agreement, or repo, market for government- backed mortgage bonds that many REITS rely on for funding is in "good" shape, it may face pressure if European banks need to retrench, American Capital President Gary Kain said.

"We see that as being a potential pressure on rates, not on availability," Kain, whose company had $43.6 billion of assets as of June 30, said today in a telephone interview. Higher rates are possible in part because the Federal Reserve is selling short-term Treasuries in its "Operation Twist" program that began this month, he said.

Kain's Bethesda, Maryland-based firm hasn't seen European banks pulling out of the market and "we have had no major line reductions," he said. "If anything over the last couple of weeks, we have had" the credit lines go up in size.

The cost of one-month repo financing for agency mortgage securities fell 2 basis points today to 25 basis points as of 12:29 p.m., according to data from ICAP Plc, the world's largest inter-dealer broker. The rate has climbed from 20 basis points on Aug. 31, tracking increases in the benchmark London interbank offered rate, and fallen from 38 basis points on Aug. 1, after a jump fueled by concern the U.S. might default on its debt.

A basis point is 0.01 percentage point.

Declines in mortgage REIT shares may also reflect concern about the companies' investments, said Richard Eckert, an analyst at B. Riley & Co. Anworth Mortgage Asset Corp. yesterday announced a dividend two cents lower than its previous quarterly payout, saying higher homeowner refinancing eroded returns, and said it plans to buy back as much as 2 million in shares.

The share-repurchases are "a signal to me they cannot find enough ARM assets at their targeted spreads and ROEs to fully deploy their capital, so they are returning it to shareholders," Eckert wrote in an e-mail, using acronyms for adjustable-rate mortgages and return on equity.

John Hillman, a spokesman for Santa Monica, California- based Anworth, said Chairman and Chief Executive Officer Lloyd McAdams wasn't immediately available to comment. Jay Diamond, a spokesman for Annaly, didn't return a message.