(Reuters) - U.S. stock index futures fell on Tuesday as worries about escalating tensions between Russia and Ukraine affected risk sentiment globally, with investors rushing to safe-haven assets.

President Vladimir Putin said Russia could consider using nuclear weapons if it was subject to a conventional missile assault supported by a nuclear power, after the United States allowed Ukraine to fire American-made long-range missiles deep into Russia.

At 5:35 a.m. ET, Dow E-minis were down 254 points, or 0.58%, U.S. S&P 500 E-minis were down 24 points, or 0.41%, and Nasdaq 100 E-minis were down 58.5 points, or 0.28%.

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

Analysts and a trader also pointed to a report from Ukrainian news agency RBC Ukraine citing a military official as saying Kyiv had carried out its first attack inside Russia with an ATACMS missile.

The CBOE Volatility index briefly jumped to its highest since the Nov. 5 U.S. election, and was last up 1.24 points at 16.79.

Futures tracking the small-cap Russell 2000 dropped 1.2%.

Shares of some U.S. defense companies gained in premarket trading, with RTX Corp and Lockheed Martin (NYSE: LMT ) up 1.8% and 1.4%, respectively.

Gold miners rose as prices of the yellow metal gained. Barrick Gold (NYSE: GOLD ) gained 2.2% and Harmony (JO: HARJ ) Gold Mining rose 4.7%.

The resurgence of geopolitical tensions comes at a time when investors assess potential policy changes from President-elect Donald Trump's administration, gauge the possible path of U.S. Federal Reserve's interest rate cuts and await quarterly earnings from AI chip firm and index heavyweight Nvidia (NASDAQ: NVDA ).

Nvidia, which reports on Wednesday, gained 0.9%.

EV maker Tesla (NASDAQ: TSLA ) lost 1.2% 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.

Retailer Walmart (NYSE: WMT ), scheduled to report results before the bell on Tuesday, is expected to post a roughly 4% rise in revenue, according to estimates compiled by LSEG. Its shares were up 1.8%.

AI server maker Super Micro Computer (NASDAQ: SMCI ) leapt 32.2% after the company named BDO USA as its auditor and said it has submitted a plan to the Nasdaq to avoid delisting.

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