The Dow Jones industrial average lost more than 500 points.
Trump took to Twitter to defend his promise from Thursday to impose stiff tariffs on imports of steel and aluminum, which has investors anxious about escalating retaliation by other countries. That means Apple and other big USA companies are dependent on customers not only in Peoria but also Paris and Peru.
The S&P 500 trimmed its loss as the day went on, and it was down 7 points, or 0.3%, at 2,670, as of 1:40 p.m.
USA stocks sank again on Wednesday and cemented February as the worst month for the market in two years. The S&P 500 Index and NASDAQ Composite traded higher by as much as 0.6 percent and 0.7 percent respectively.
The Canadian dollar was trading at 77.57 cents United States, down 0.24 of a USA cent.
About 8.1 billion shares changed hands on USA exchanges.
Energy shares dropped with oil prices CLc1 and the sector had the biggest daily decline in the S&P 500, but a break below the 50-day moving average on the S&P 500 triggered further selling in afternoon trading.
The Institute for Supply Management said its manufacturing index rose to 60.8% in February, up from 59.1% in January, to reach the highest level since May 2004.
Celgene fell about 7 percent after USA health regulators rejected the company's application seeking approval of a key multiple sclerosis drug.
Earlier, shares of the steelmakers swung wildly amid speculation that the US government would impose tariffs or quotas on steel and aluminum imports.
Worries about the possibility of much higher interest rates have been at the centre of the market's troubles over the last month.
But some investors speculated it could mean the central bank will get more aggressive in raising interest rates.More news: Rose raring to go at WGC in Mexico
Powell stressed that he did not want to prejudge the new set of projections, but his comments still raised concerns about four rate increases this year.
There could be more fireworks in the market on Thursday.
Total revenue rose 20 per cent on a year ago to $107 million.
Inc.'s stock (CRM) rose 4.7% after the software company's quarterly results beat expectations (http://www.marketwatch.com/story/salesforce-shares-wobbly-as-results-outlook-top-street-view-2018-02-28).
The month also included a confirmation of a 10-percent correction for the stock market but that was followed by a rebound that made the monthly losses less steep.
A monthly loss for the S&P 500 and the Dow Industrials would be their first fall since March of a year ago.
Shares of steel companies AK Steel and US Steel Corp gained on signs of a possible major trade announcement by President Donald Trump, who has been considering imposing hefty tariffs on imports of the metals from China and other countries. On the Nasdaq, 1,577 issues rose and 628 fell.
Dick's Sporting Goods rose 1 percent.
The dollar dipped to 106.40 from 106.66 yen late Wednesday. The euro fell to $1.2179 from $1.2202.
European markets closed mostly lower: London's FTSE 0.1 per cent, Paris closed flat and Frankfurt was 0.3 per cent lower.
In Asia, Japan's Nikkei 225 plunged 2.5 per cent, the Hang Seng in Hong Kong fell 1.5 per cent and South Korea's Kospi dropped 1 per cent.