(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 up 1.8% and 1.4%, respectively.

Gold miners rose as prices of the yellow metal gained. Barrick Gold gained 2.2% and Harmony 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.

Nvidia, which reports on Wednesday, gained 0.9%.

EV maker Tesla 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, 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 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.