Also, fears about the sector's indebtedness are overblown, Commerzbank corporate debt strategist Apostolos Bantis said.

He estimates emerging companies' ratio of debt to EBITDA -- earnings before interest, depreciation and amortization -- at 2.5-3.0 percent on average, is a similar level to U.S. investment grade firms and half that of junk-rated U.S. issuers.

"Fundamentally, emerging market corporates are in better shape than U.S. high-yield and are comparable with U.S. investment-grade," Bantis said.

Tight supply, an added factor in the rally, could also change soon because both emerging market companies and sovereigns face "a wall of maturity" as the huge debt volumes raised during the boom times come due for repayment. About $1.6 trillion falls due in the coming five years.

Added to that, new bonds have been scarce this year, with gross issuance not seen exceeding $220 billion, JPMorgan says, down from $239 billion last year and $372 billion in 2014.

This is exacerbated by companies rolling over debt and buying back bonds. As a result, net issuance may total just $102 billion this year, JPMorgan estimates.

However, Aviva Investors' deputy head of emerging market debt, Aaron Grehan, whose firm has nearly $415 billion in assets under management, sees no risk of a bubble and says that crossover investors' appetite should encourage more issuance.

"The spread tightening we have experienced is likely to trigger greater issuance versus expectations," he said.

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