(Reuters) - U.S. stock index futures plummeted on Monday, and the S&P 500 looked set to confirm a bear market, as investors barreled into government bonds on economic worries over the fallout of U.S. President Donald Trump's sweeping tariff plans.

The 10-year U.S. Treasury yields fell to 3.953%, with investors pricing in a chance of a fifth interest-rate cut from the Federal Reserve this year.

S&P 500 futures plunged more than 20% from their peak, suggesting the benchmark index has been in a bear marketsince February if it closes down 20% from its all-time highs.

Trump told reporters late on Sunday that investors must endure the consequences and that he would refrain from negotiating with China until the U.S. trade deficit is addressed.

In the two sessions after Trump's tariff decision, the index has tumbled 10.5%, erasing nearly $5 trillion in market value, marking its most significant two-day loss since March 2020.

By 4:42 a.m. ET (0942 GMT) U.S. S&P 500 E-minis were down 159 points, or 3.11%, Nasdaq 100 E-minis were down 598.5 points, or 3.41%, Dow E-minis were down 1,178 points, or 3.06%.

The CBOE Volatility Index, seen as Wall Street's fear gauge, was up 7.57 points at 52.88.

The sharp declines in the last two sessions pushed the tech-heavy Nasdaq into bear market, while the Dow Jones Industrial Average slumped more than 10% from its record-closing high.

The fear of a tariff-led recession caused markets to bring into play the chances of an interest-rate cut in May, with traders seeing a 54% possibility.

Meanwhile, the week ahead is packed with a series of economic indicators, with consumer price data set to take center stage on Thursday.

(Reporting by Pranav Kashyap in Bengaluru; Editing by Arun Koyyur)