Janet Yellen, chair of the Federal Reserve, said policymakers should be careful not to "move too slowly" on monetary policy, despite "significant uncertainty" over inflation.

Yellen on Tuesday in Cleveland, Ohio the National Association for Business Economics (National Association for Business Economics) to speak at the annual meeting, admitted that "support the baseline outlook of some of the key assumptions may be wrong, and this means that compared to the current forecast, inflation will be at the low end longer."

But she opposed waiting too long before raising rates again, reinforcing the fed's signal at last week's latest policy meeting that it could raise interest rates again by the end of 2017.

"In low inflation, neutral under the condition of low real interest rates, a gradual approach is particularly appropriate, adversely impact, this means that if the economy (the federal open market committee cut the federal funds rate of space will be limited," she said.

"But we should also be wary of moving too slowly.

Job growth continues to maintain a good momentum, far ahead of our estimated and average long-term pace of employment for new entrants to the workforce."

She said, if not with the passage of time further moderately raised the federal funds rate, the Labour market there is a risk of overheating, probably sometime in the future have an inflation problem, and the problem may be hard to overcome on the premise of without causing a recession.

The cause of persistently low inflation has become a focal point of the heated debate among fed rate-setters.

Some of ms yellen's colleagues have come up with some theories, including structural suppression of price levels by technology.

Ms yellen has pointed to other causes of low inflation over the past few years, including a surplus in the job market after the recession, lower energy prices and a stronger dollar.

She said on Tuesday that more special factors, including a drop in the price of telecommunications services, meant that the recent drop in inflation was "likely to be temporary".

Yellen speaking, is considered the most sensitive in monetary policy on two-year Treasury yields of 1.45%, this is the highest since 2008, higher than last week, shortly after the fed meeting reached a peak.

The dollar index, which measures the value of the dollar against a basket of currencies, initially gave up some of the gains, then recovered to 93.16, and the shares were basically stable.