US Federal Reserve Chairman Jerome Powell has tried to reassure financial markets that rising interest rates were not locked in and reasserted the central bank's independence, saying he would not resign even if US President Donald Trump asked him to.

Mr Trump named Mr Powell to the helm of the Fed at the start of the year but has been a frequent and vocal critic, blaming Mr Powell and the Fed for raising rates which he says pose a threat to his economic agenda - an unprecedented public berating that breaks with recent norms.

Asked at an economic conference in Atlanta if he would step down should Mr Trump request his resignation, Mr Powell said: "No."

He said he had not heard directly from the US President, despite Mr Trump's many recent Twitter outbursts, and Mr Powell repeated that the institution remained outside political considerations.

"People should know the Fed has a very strong culture around non-political activity and we are committed to achieving the goals the law gives us in a completely non-political way based on the best thinking," Mr Powell said.

US and global stock markets have tumbled in recent weeks and 2018 was Wall Street's worst year since the global financial crisis amid worries about the slowing world economy, trade tensions and rising interest rates.

Mr Trump's attacks have added to the jitters, since it could cause the Fed to tighten policy just to prove its independence.

But Mr Powell once again stressed that the Fed had no "pre-set" plan for interest rates and would bide its time to see how the economy evolves.

The Fed's policy committee raised the benchmark lending rate four times last year but signalled last month it expected only two increases this year, rather than three.

There is "no pre-set path for policy and particularly with the muted inflation themes coming in we will be patient as we see how the economy evolves," Mr Powell said.

The Fed is "prepared to adjust policy quickly and flexibly" to support the economy, he added.

Mr Powell remained upbeat about US economic prospects in the near future, pointing to the strong data - including a blockbuster jobs report for December released today - but noted financial markets were worried about a slowdown in the US and Chinese economies.

The sharp decline in manufacturing sentiment in December prompted Wall Street to drop sharply on Thursday but it recovered most of that lost ground before midday following the big jump in job creation and after Mr Powell's reassuring comments.

He also said the 3.2% jump in wages in 2018 did not set off alarm bells for inflation, reinforcing the clear signal that policymakers were in no hurry to clamp down on rising prices.

"We're listening carefully... to the message that markets are sending and we'll be taking those downside risks into account as we make policy going forward," he said.