The US Federal Reserve is to trim its aggressive bond-buying programme, chairman Ben Bernanke has announced.

In what amounts to the beginning of the end of its unprecedented support for the US economy, the central bank said it would reduce its monthly asset purchases by $10 billion to total $75 billion.

It trimmed equally from mortgage and Treasury bonds.

The move, which could come as a surprise to many investors, was a nod to better prospects for the economy and labour market and marks a historic turning point for the largest monetary policy experiment ever.

The Fed's asset purchase programme, a centrepiece of its crisis-era policy, has left it holding roughly $4 trillion of bonds, and the path it must follow in dialling it down is rife with numerous risks, including the possibility of higher-than-targeted interest rates and a loss of investor confidence.

The Fed "modestly" reduced the pace of bond buying in light of better labour market conditions, it said in a statement following a two-day policy meeting.

But in a move likely meant to forestall any sharp market reaction that could undercut the recovery, the central bank also said it "likely will be appropriate" to keep rates near zero "well past the time" that the jobless rate falls below 6.5%.

It was a noteworthy tweak to a previous commitment to keep benchmark credit costs steady at least until the jobless rate hit 6.5%. The rate stood at 7% in November, a five-year low.

The Fed's latest so-called quantitative easing programme, or QE, was launched 15 months ago to kick-start hiring and growth in an economy that was only slowly recovering. 

The Fed's first QE programme was launched in the midst of the 2008 financial crisis.

Meanwhile, the Fed lowered its expectations for both inflation and unemployment over the next few years, acknowledging the faster-than-expected drop in joblessness to a five-year low of 7% last month. 

It expects the unemployment rate to fall to 6.3-6.6% by the end of 2014, from a previous prediction of 6.4-6.8%, according to the central tendency of policymakers.

Three policymakers now expect the first rate rise to come in 2016, up from only two making that prediction in September, while a strong majority of 12 officials still see the move in2015.

The Fed has kept interest rates near zero since the depths of the financial crisis in late 2008 and asset purchases have stoked anxiety that they could unleash inflation or fuel hard-to-detect asset price bubbles.

Even some within the Fed have worried the bond purchases could have unintended and economic costly effects.

The unprecedented money-printing has helped drive US stocks to record highs and sparked sharp gyrations in foreign currencies, including a drop in emerging markets this year as investors anticipated an end to the easing.

Earlier, Brazil's finance minister issued a plea for the Fed to end its buying sooner rather than later to reduce market uncertainty that has kept emerging economies on edge.

But some have credited the Fed's asset purchases with stabilising an economy and banking system that had been crippled by the 2008 financial crisis and with staving off what could have been a damaging cycle of deflation.

Recent growth in jobs, retail sales and housing, as well as a fresh budget deal in Congress, had convinced a growing number of economists the Fed would trim the bond purchases. 

The 15-month-old programme is meant to put downward pressure on long-term borrowing costs to stimulate investment and hiring.

But many thought the central bank would wait until early in the new year, given persistently low inflation and the fact that the world's largest economy has stumbled several times in its crawl out of the 2007-2009 recession.

A handful of the Fed's policymakers had been pushing for the US central bank to better telegraph how it plans to wind down the stimulus programme, or to clarify its longer-term intentions to keep policy loose.

The Fed policy meeting was the penultimate one of chairman Ben Bernanke's tenure.

His second four-year term as chairman of the central bank expires on 31 January, just two days after the close of the Fed's first policy meeting of 2014.