U.S. stock index futures fell as concerns over escalating tensions between Russia and Ukraine affected risk appetite globally, with investors rushing to safe-haven assets.

Russian President Vladimir Putin issued a warning to the United States on Tuesday, lowering the threshold for a nuclear strike just days after the administration of Joe Biden reportedly allowed Ukraine to fire American missiles deep into Russia.

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 Russian Defence Ministry.

"The (equity declines) are Europe-led, and that's what is moving the U.S. futures down as well," said David Morrison, senior market analyst at Trade Nation.

But the declines were a "knee-jerk" reaction as the outcome of Putin's threats were unclear, Morrison said.

The CBOE Volatility index briefly jumped to its highest since the Nov. 5 U.S. election, and was last up 1.27 points at 16.85.

At 07:05 a.m. ET, Dow E-minis were down 232 points, or 0.53%, S&P 500 E-minis were down 18.25 points, or 0.31% and Nasdaq 100 E-minis were down 38.5 points, or 0.19%.

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 1.7% and Harmony Gold Mining rose 4.3%.

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

Retailer Walmart gained 3.2% on Tuesday 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.

Shares of some U.S. defense companies gained in premarket trading, with RTX Corp and Lockheed Martin up 1% and 1.1%, respectively.

Nvidia, which reports third-quarter results on Wednesday, gained 1.2% in premarket trading.

EV maker Tesla lost 0.7% after sharp gains in the previous session following a report on Monday that Trump's transition team was planning to set up federal regulations for autonomous vehicles.

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

AI server maker Super Micro Computer leapt 20.4% 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 1.7% despite forecasting a slower-than-expected drop in annual comparable sales.

Incyte fell 12% after the drugmaker paused enrollment in an ongoing early-phase study on its skin condition treatment.