U.S. stock index futures fell on Tuesday after a slide in Apple Inc, which had its weighting cut in a rebalancing of shares in the Nasdaq 100, forcing some to sell the iPhone maker's stock.

Once the rebalancing is effective on May 2, the projected weight of Apple Inc will be 12.33 percent of the index compared with its current 20.49 percent, Nasdaq OMX said on its website.

The rebalancing requires sponsors of exchange traded funds which track the Nasdaq 100 <.NDX>, like the heavily traded PowerShares QQQ Trust to also lower their holdings of Apple shares. Apple shares fell 1.5 percent in premarket trading.

Microsoft Corp's weighting in the index will rise, and its shares gained 2.7 percent before the open.

Other tech stocks in focus include Texas Instruments after it launched a $6.5 billion takeover bid for National Semiconductor Corp , offering a 78 percent premium. National Semi traded up 71.6 percent at $24.15 premarket.

S&P 500 futures fell 4.7 points and were below fair value, a formula that evaluates pricing by taking into account interest rates, dividends and time to expiration of the contract. Dow Jones industrial average futures lost 31 points and Nasdaq 100 futures dropped 20.25 points.

Oil prices hovered near their highest levels since 2008 on Tuesday, with Brent near $121 a barrel. Oil prices were supported by the unrest in the Middle East and North Africa as well as delays in elections in Nigeria.

Investors will also look closely at the minutes from the latest meeting of the Federal Reserve's policy-setting committee. Federal Reserve Chairman Ben Bernanke said a recent rise in U.S. inflation was driven primarily by rising commodity prices globally and was unlikely to persist.

The S&P 500 met tough resistance on Monday, failing to break a level that has held since mid-February. It ended flat even as a spate of deals and underlying strength in the economy spurred optimism.

About 5.94 billion shares traded on the New York Stock Exchange, NYSE Amex and Nasdaq, the lowest total of the year.

(Reporting by Rodrigo Campos; Editing by Kenneth Barry)