U.S. stock index futures fell on Monday as a resurgent U.S. dollar sparked a pullback in global commodity prices and investors sought more definitive signs to justify the market's six-month run-up.

Futures suggested that Wall Street would open down about half a percent or more, with shares of energy companies and miners among the notable drags. The energy sector exchange-traded fund shed 1.2 percent before the bell, and crude oil futures fell 2.1 percent to $70.54 a barrel.

The focus right now is a little bit on the dollar, said Rick Meckler, president of investment firm LibertyView Capital Management in New York.

Now that the dollar is rallying a little bit, it's pushing some technical traders to take some profits (in stocks). I think that's probably the biggest single factor this morning.

S&P 500 futures dipped 5.30 points and were below fair value, a formula that evaluates pricing by taking into account interest rates, dividends and time to expiration on the contract. Dow Jones industrial average futures were off 49 points, while Nasdaq 100 futures dipped 9.75 points.

Computer maker Dell Inc announced a $3.9 billion proposed takeover of Perot Systems Corp

, a deal likely to underscore investors' hopes that stock valuations remained attractive even after the market's strong run-up from early March.

The boost from that news, however, was tempered by the rising U.S. dollar. The dollar index, which measures the greenback against a basket of major currencies, rose 0.5 percent. <.DXY>

The benchmark S&P 500 <.SPX> is up 58 percent since hitting a 12-year closing low in early March, partly because of strong second-quarter earnings and optimism that an economic recovery is gaining traction.

That optimism is beginning to come under some strain, however, as investors seek more clarity about the 2010 profit outlook and await hints of how strong results will be for the rest of the year.

(Reporting by Ellis Mnyandu; editing by Padraic Cassidy)