The United States Federal Reserve has raised the benchmark interest rate for the third time this year but showed no indication it would be more aggressive in efforts to head off inflation.

The central bank boosted the lending rate to 2.0%-2.25%, a quarter-point increase in the target range.

It once again said "further gradual increases" would allow continued expansion of the economy while keeping inflation around the Fed's 2% target.

However, the closely scrutinised statement from the rate-setting Federal Open Market Committee removed a longstanding phrase which described the Fed's rate stance as providing stimulus to the economy.

In a statement US Fed Chairman Jerome Powell explained the thinking behind the unanimous decision.

"The committee expects that further gradual increases...will be consistent with sustained expansion of economic activity, strong labour market conditions and inflation near the committee's symmetric 2% objective over the medium term," the statement said.

But the statement no longer describes the rate level as "accommodative," while continuing to note the strong gains in economic activity and employment.

Mr Powell said the US central bank does not take "political factors" into consideration when deciding whether to raise interest rates. 

He downplayed US President Donald Trump's recent comments objecting to moves he said could undermine the booming economy. 

The Fed members "try to set monetary policy to achieve maximum employment in a complex of price stability.... We don't consider political factors or things like that," he said. 

The Fed has now raised the key rate eight times since late 2015, with one more expected in the final meeting of the year in December.

In their quarterly forecast, the central bankers did not signal that they would have to act more aggressively to constrain prices, as the inflation forecast for next year was slightly lower than in June, while the interest rate outlook was unchanged.

The median forecast continues to expect the federal funds rate to end the year at 2.4%, implying one more rate hike - as is widely expected in December.

The rate is seen rising to 3.1% in 2019 - the same as the last forecast - which indicates three more moves, as inflation is expected to hold at the Fed's goal of 2%.

The Fed's preferred price index, based on consumer spending, rose 2.3% in the 12 months ended in July, the strongest since March 2012.

But excluding volatile food and energy prices, the inflation rate rose 2.0%, which hits the Fed's goal.