U.S. stock index futures indicated a drop of more than 1 percent at the open on Tuesday as a slowdown in global manufacturing added to doubts about the pace of an economic recovery.

Investors also worried about over the euro zone sovereign debt crisis after Spain's credit rating was downgraded by Fitch last week. Adding to concerns, the European Central Bank said banks in the region face another $239 billion in potential writedowns.

A survey showed manufacturing activity in the euro zone expanded in May at a considerably more sluggish pace than in April, while separate data indicated the pace of China's factory output eased last month.

Meanwhile, influential economist Nouriel Roubini said advanced economies face years of anemic growth and the risk of a double-dip recession as they cope with a weak labor market and high debt.

U.S. economic indicators on tap for Tuesday include April construction spending data and the Institute for Supply Management's May manufacturing index, both due at 10:00 a.m.

April construction spending is expected to remain flat, compared with 0.2 percent growth in the prior month. The Institute for Supply Management's May manufacturing index is seen at 59 in May, down from April's reading of 60.4.

American International Group Inc shares fell 3.2 percent to $34.25 in light premarket trading after it rejected a lower offer from Prudential Plc for its Asian division.

S&P 500 futures fell 15 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 sank 113 points, and Nasdaq 100 futures lost 18.5 points.

Crude oil futures were down 2.5 percent as the manufacturing data renewed concerns about demand.

U.S.-listed shares of BP Plc dropped 15 percent to $36.45 premarket following its failed attempt to stem the Gulf of Mexico oil spill. BP stock has lost more than one-third since the massive spill started six weeks ago.

Apple Inc has sold 2 million of its iPad touch-screen tablet computer since the U.S. launch nearly two months ago and the unveiling in nine international markets this past weekend. The stock rose 0.2 percent at $257.51 premarket.

U.S. stocks fell Friday, capping their worst month in over a year as a Fitch downgrade of Spain's credit rating reignited worries about euro-zone debt.

(Editing by Jeffrey Benkoe)