"The rupee is responding to market pressures," Central Bank governor Nivard Cabraal said.
"When the dollar is weakening, we will probably see the rupee appreciating and if the dollar strengthens, you will see the rupee depreciating against it.
"So that kind of a fluctuation could occur."
Sri Lanka has been easily maintaining a peg from around April by keeping policy rates steeply above the US rate by steadily draining liquidity from markets at 8.0 percent compared to near zero levels in the US.
In the past few decades, despite having high policy rates compared to the US, the Central Bank has come under Treasury pressure to print quantities of money for state expenditure - a type of quantity easing - which de-stabilized the peg and brought high inflation.
But over the past year inflation has been in low single digits.
The Central Bank is expecting inflation to end the year at 3.5 percent.
Cabraal said it was too early to say whether the central bank will use the exchange rate to counter dollar generated inflation in the way the Singapore Monetary Authority adjusts the exchange rate, though the US dollar weakening was "concern" for inflation.
"We will watch the whole situation a little more carefully to see how it emerges," Cabraal said.
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"And may be it won't be just a straight line - not just an exact peg that we have - which people have been suggesting should not be also.
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"Because I think we are allowing market forces also to come back and move the prices as much as possible."
We will watch the whole situation a little more carefully to see how it emerges. And may be it won't be just a straight line - not just an exact peg that we have - which people have been suggesting should not be also.
Sri Lanka has dual 'anchors' for inflation, in the form of an external anchor which is the peg, and a domestic inflation target of 'low single digits' for which policy rates are also used.
Singapore however does not formally control interest rates.
Sri Lanka's dual anchors as well as flaws in its monetary law which forces it to finance the government have been blamed by economists for the poor inflation record of the Central Bank in the past and general monetary and economic instability of the country.
A frequently adjusting peg, unless it has direct monetary policy benefits, can be a 'division' for trade with the rest of the world, bringing profits to banks but raising costs to importers and exporters.
Updated