U.S. stock indexes opened shartply lower Tuesday, as concerns over escalating tensions between Russia and Ukraine dented risk appetite globally, with investors rushing to safe-haven assets.

At 9:42 a.m. EST, the Dow was down by 361.45 points, or 0.83%, the S&P 500 had given up 26.57 points, or 0.45%, and the Nasdaq 100 had shed 32.88 points, or 0.17%.

Russian President Vladimir Putin lowered the threshold for a nuclear strike in response to a broader range of conventional attacks, and Moscow said Ukraine had struck deep inside Russia with U.S.-made ATACMS missiles.

Ukraine launched an attack on the Russian Bryansk region overnight with six U.S.-made long-range missiles, Russian news agencies reported on Tuesday, citing the Russian Defence Ministry.

"All around the world, we're nervous because Putin has said if this escalates, he could match the escalation. Markets are reflecting this nervousness," said Kim Forrest, chief investment officer, Bokeh Capital Partners.

The CBOE Volatility index briefly jumped to its highest at 17.88, since the Nov. 5 U.S. election, before slightly easing to 16.61.

The resurgence of geopolitical tensions added jitters during a crucial week for Wall Street, as investors await key cabinet appointments for President-elect Donald Trump's administration, assess the possible path for interest rate cuts and await quarterly earnings from AI giant Nvidia.

Nvidia, which reports third-quarter results on Wednesday, gained 0.8% in premarket trading. Given its weight in the main indexes, its results could offer equities support in a volatile market.

"The combination of Russia ratcheting up its war rhetoric and uncertainty about how the incoming U.S. presidential administration will respond, is a recipe for stock market volatility," said Gaurav Mallik, chief investment officer, Pallas Capital Advisors.

Investors rushed to safe-haven assets including government bonds, gold and the Japanese yen.

Gold miners rose as prices of the yellow metal gained. Barrick Gold gained 1.9% and Harmony Gold Mining rose 4.9%.

Futures tracking the small-cap Russell 2000 dropped 0.8%.

But most investors have maintained fairly upbeat forecasts for equities - both Goldman Sachs and Morgan Stanley have forecast the S&P 500 would reach 6,500 by the end of 2025.

The Nasdaq and the S&P 500 closed higher on Monday.

Retail giant Walmart gained 3.2% after it raised its annual sales and profit forecasts for the third consecutive time, a sign that it may be gaining market share ahead of the holiday season.

Some U.S. defense companies gained in premarket trading, with RTX Corp and Lockheed Martin up marginally.

AI server maker Super Micro Computer leapt 25.1% after the company named BDO USA as its auditor and said it has submitted a plan to the Nasdaq to avoid delisting.

Home improvement retailer Lowe's lost 2.2% despite forecasting a slower-than-expected drop in annual comparable sales.

Incyte fell 10.9% after the drugmaker paused enrollment in an ongoing early-phase study on its skin condition treatment.

Kansas Fed president Jeffrey Schmid is scheduled to speak later in the day.