Wall Street stocks tumbled again Tuesday as initially successful efforts to rebound from big losses faded amid worries over US President Donald Trump's trade wars.

All three major indices finished firmly in the red, with the S&P 500 shedding 1.6 percent to 4,982.77, its first close below 5,000 points in nearly a year.

The Dow Jones Industrial Average ended down 0.8 percent at 37,645.59, a swing of about 1,780 points below its session peak, while the tech-rich Nasdaq Composite Index shed 2.2 percent to 15,267.91.

After three days of losses, Wall Street stocks opened buoyantly as traders embraced talk of White House negotiations with Japan and South Korea in the hopes Trump's trade onslaught might be short-lived.

But investors grew edgy as the day progressed with no concrete progress. Meanwhile, the White House confirmed plans for massive tariffs on China to go into effect overnight.

"Obviously investors are clamoring for clarity and there still isn't any," said Jack Ablin of Cresset Capital, who estimated that the market now sees a greater than 50 percent chance of a US recession.

Stocks have been in free fall since Trump's "Liberation Day" event announcing tariffs on major US trading partners last Wednesday.

White House officials have signaled openness to dealmaking while blasting China for enacting sharp retaliatory tariffs in response to the new US levies.

Trump plans to impose another 50 percent duty on Chinese goods at midnight, bringing the additional rate on Chinese products to 104 percent.

All 11 sectors in the S&P 500 finished the day in the red, while most of the Dow was also in negative territory. 

Especially big losers in the blue-chip index included Apple, down five percent and Nike, down 4.2 percent.

But UnitedHealth Group surged 5.4 percent after the Trump administration finalized a rule boosting payments to insurers under the government Medicare program.