Wall Street was setting up for a tough session on Monday, with U.S. stock futures under intense pressure as investors moved out of perceived riskier assets following a weekend of escalating tensions over Russia and Ukraine.

Investors switched from stocks to commodities, the dollar and bonds as headlines commanded the action. They also had to contend with data: consumer spending, a manufacturing index and auto sales.

With losses building ahead of the open, futures for the S&P 500 index fell 20.9 points, or 1.1%, to 1,836.70, while those for the Dow Jones Industrial Average tumbled 157 points, or 1%, to 16,1505. Futures for the Nasdaq-100 index dropped 41.75 points, or 1.1%, to 3,653.50.

There is a handful of data to get through. Consumer spending for January is expected to rise 0.2% from a 0.4% rise in the prior month, with that data due at 8:30 a.m. EST.

The Institute of Supply Management's February manufacturing index is due at 10 a.m. EST, while the Markit PMI index is expected an hour earlier. Car makers will be in focus on the release of February auto-sales reports, which are forecast to be downbeat due to harsh wintry weather. Another storm is moving through the East Coast on Monday.

Otherwise, for investors, Monday will be all about geopolitical tensions and watching the headlines, said strategists.

When times are uncertain, buyers back away because they know short sellers, who know they can move the market easily, are willing to make bigger bets, said Chris Weston, chief market strategist at IG, in a note. U.S. stocks finished mixed on Friday, but ended February with strong monthly gains, and the S&P 500 notched its 48th record close for the past year.

"Invariably, many will be buying these dips, but on an index level I would stand aside and wait for clarity to develop, as you know this market can go lower, very quickly, if geopolitics deteriorates and U.S. data doesn't show any signs of a pickup," Mr. Weston said.

A senior U.S. official told The Wall Street Journal on Sunday that Russian forces had "complete operational control of the Crimean Peninsula," and Ukraine mobilized troops over the weekend. Some 6,000 Russian ground and naval forces are reportedly in the region and more reinforcements are expected to come. Group of Seven leaders condemned Russia's "clear violation of the sovereignty and territorial integrity of Ukraine," saying preparation for the planned Group of Eight summit in Sochi, Russia is on hold.

Russian stocks and currency came under intense pressure, and the country's central bank raised its key interest rate to 7% from 5.5% in a bid to support the ruble, which fell more than 1.8% against both the dollar and the euro. Russia's blue-chip MICEX index fell nearly 11% in Moscow.

The flight to safety pushed the yield on the 10-year U.S. Treasury lower by 2%, or five basis points, to 2.595%. April gold jumped $23.30, or 1.8%, to $1,344.80 an ounce, a level not seen since October for a front-running contract, according to FactSet Research. Crude oil for April delivery jumped $1.70, or 1.7%, to $104.29 a barrel, levels not seen since September, according to FactSet Research.

European stocks suffered sharp losses, with the main Stoxx Europe 600 index tumbling 2.2%. Most Asian markets fell, outside of the Shanghai Composite Index, which gained nearly 1%. Both the official and HSBC versions of China's monthly manufacturing survey showed the sector was weaker last month than in January. The HSBC purchasing managers index fell to 48.5 from 49.5, but was above the preliminary version from last month.