The US Federal Reserve has kept interest rates unchanged at 0-0.25% in a nod to concerns about a weak world economy, but left open the possibility of a modest policy tightening later this year.

In what amounted to a tactical retreat, the US central bank said an array of global risks and other factors had convinced it to delay what would have been the first rate hike in nearly a decade.

"Recent global economic and financial developments may restrain economic activity somewhat and are likely to put further downward pressure on inflation in the near term," the Fed said in its policy statement following the end of a two-day meeting.

It added the risks to the US economy remained nearly balanced but that it was "monitoring developments abroad."

However, the central bank maintained its bias towards a rate hike sometime this year, while lowering its long-term outlook for the economy.

Fresh economic projections showed 13 of 17 Fed policymakers still foresee raising rates at least once in 2015, down from 15 at the last meeting in June. Four policy makers now believe rates should not be raised until at least 2016, compared to two who felt that way in June.

The Fed has policy meetings in October and December.

In deciding when to hike rates, the Fed repeated that it wanted to see "some further improvement in the labour market," and be "reasonably confident" that inflation will increase.

Taken as a whole, the latest Fed projections of slower GDP growth, low unemployment and still low inflation suggest that concerns of a so-called secular stagnation may be taking root among Fed policymakers. One policymaker even suggested a negative federal funds rate.

The median projection of the 17 policymakers showed the Fed expects the economy to grow 2.1% this year, slightly faster than previously thought. However, its forecasts for GDP growth in 2016 and 2017 were downgraded.

Policymakers also forecast inflation to creep only slowly toward the Fed's 2% target even as unemployment dips lower than previously expected. They now expect the unemployment rate to hit 4.8% next year, remaining at that level for as long as three years.