Wall Street's biggest firms are predicting intensifying bond losses in emerging markets, where borrowing costs have already soared to the highest level in more than four years versus U.S. corporate debt, as the Federal Reserve considers curtailing record stimulus.

“We're not yet convinced that we've seen the worst in terms of flows out of emerging markets,” Jeffrey Rosenberg, the chief investment strategist in fixed-income at New York-based BlackRock Inc., the world's largest asset manager, said in a telephone interview, expressing his own views. “We see a lot of valuation change but we see the potential for even more valuation change.”

Investors have yanked $22.1 billion from emerging-market bond funds since the end of April, almost five times the amount pulled from U.S. corporate credit, according to EPFR Global. That's pushed the extra yield that buyers now demand to own dollar-denominated emerging-market debt instead of U.S. company notes to 1.4 percentage points, about the most since December 2008.