US President Donald Trump has blasted the Federal Reserve, describing it as the "only problem" for the US economy, as top officials convened to discuss a growing rout in stock markets caused in part by the president's attacks on the central bank.

Stocks have fallen again amid concern about slowing economic growth, the government shutdown and reports that Mr Trump had discussed firing Federal Reserve Chairman Jerome Powell.

US stocks have dropped sharply in recent weeks on concerns over weaker economic growth, with the S&P 500 index on pace for its biggest percentage decline in December since the Great Depression.

In a tweet that did nothing to ease market concerns about the Fed's cherished independence, Mr Trump laid the blame for economic headwinds firmly at the feet of the central bank.

"The only problem our economy has is the Fed. They don't have a feel for the market," Mr Trump said on Twitter.

"The Fed is like a powerful golfer who can't score because he has no touch - he can't putt!"

Mr Trump has frequently criticised the Fed's raising of interest rates this year and has gone after Powell several times now, telling Reuters in August he was "not thrilled" with his own appointee.

The Fed hiked rates again last week.

A crisis call between US financial regulators and the Treasury Department failed to assure markets.

All three major indexes ended down more than 2% on the day before Christmas.

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The S&P 500 ended down about 19.8% from its 20 September closing high, just shy of the 20% threshold commonly used to define a bear market.

Oil prices followed equities down, tumbling more than 6% to the lowest in over a year.

US Treasury Secretary Steven Mnuchin hosted a call with the president's Working Group on Financial Markets, a body known colloquially as the "Plunge Protection team," which normally only convenes during times of heavy market volatility.

Regulators on the call said they not seeing anything out of the ordinary in financial markets during the recent sell-off and also discussed how they will continue critical operations during the partial government shutdown, according to two sources familiar with the matter.

Mr Mnuchin also made calls to top US bankers on Sunday and got reassurances that banks were still able to make loans, the Treasury said.

Wall Street is also closely following reports that Mr Trump had privately discussed the possibility of firing the Fed's Powell.

Mr Mnuchin said on Saturday that Mr Trump told him he had "never suggested firing" Mr Powell.

Still, just the public suggestion that Mr Trump might try to interfere so deeply with the Fed was unsettling to financial markets that have long operated on the presumption of the US central bank's independence from political meddling.

Adding to that disquiet is the lack of clarity over whether Mr Trump could in fact dismiss Mr Powell.

Troubles in Washington have escalated in recent days with a partial government shutdown that began on Saturday following an impasse in Congress over funds for a wall on the border with Mexico.

Defense Secretary Jim Mattis' resignation on Thursday after Mr Trump's surprise decision to pull US troops out of Syria also unsettled investors.

But Mr Mnuchin's response to the market concerns was seen as an overreaction in Wall Street circles.