Foreign-exchange reserves are emerging as the latest battleground between traders and developing nations trying to stem the worst rout in their currencies since 2008.

Nations with the smallest reserves to fend off currency speculators will continue to see their exchange rates under pressure, options prices show. Of the 31 major currencies tracked by Bloomberg, traders are most bearish on Argentina's peso, Turkey's lira, Hungary's forint, Indonesia's rupiah, and South Africa's rand, while the forwards market signals that Ukraine's hryvnia will fall 20 percent in a year.

"If you start to burn too quickly through your foreign reserves, it's an ominous sign—and of course in the forex market, they smell blood," Robbert Van Batenburg, the director of market strategy at broker Newedge Group SA in New York, said Feb. 5 by phone. "It creates this domino effect."