The Mexican peso gained 0.2 percent while the Chilean peso rose 0.4 percent less than an hour before the Fed's policy announcement, which most investors believe to include a third round of its bond-buying program.

By injecting money into the economy, the Fed's program is expected to drive large dollar flows into higher-yielding emerging market economies, further strengthening their currencies.

The Brazilian real was unchanged at 2.0256 per dollar, however, as investors bet the central bank would intervene again to keep the currency above the level of 2 per dollar, which the government sees as beneficial to exporters.

"The currency moves are driven by expectations of the Fed announcement. Most players believe the Fed will act, most likely by launching QE3," said Luiz Henrique de Paula, manager of the currency desk at H.Commcor brokerage in Sao Paulo.

"On the other hand, we have a strong defense of the 2-per-dollar floor by the Brazilian central bank, which has clearly showed it won't allow the currency to strengthen past this level."