The broad S&P 500 index is down 1.1 percent, the Dow Jones Industrial Average is down 1.2 percent, and the tech-heavy Nasdaq Composite index is down 1.6 percent.
U.S. WTI oil has risen 13 percent during the day, now trading at $113.2 per barrel. North Sea oil has risen just over 8 percent to $109.3 per barrel.
The price of oil had fallen back slightly and stock markets had risen in recent days on hopes that the war between the U.S., Israel and Iran was about to end.
But overnight on Maundy Thursday, local time in Sweden, pessimism once again took over when Trump, in his address to the nation, threatened "extremely harsh" attacks in the coming weeks.
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