Stock index futures pointed to a lower open on Tuesday as ongoing concerns over the strength of an economic recovery looked set to eclipse a report expected to show U.S. manufacturing expanded in August.

After a 50-percent run up in the S&P 500 index since early March, analysts said investors were pausing heading into September, a traditionally weak month for stocks, to assess whether the gains have been justified by economic fundamentals.

The Institute for Supply Management's survey of manufacturing follows similar reports in China and Europe that showed manufacturing around the world continues to stabilize. Investors will also look to pending home sales data later this morning for signs the nascent housing recovery is gathering steam.

The ISM is a big number in that its going to validate further improvement in the manufacturing sector, said Dan Greenhaus, analyst at Miller Tabak & Co in New York. But at the same time the strength in the regional manufacturing indexes has to an extent already told us this.

September is the worst month for the Dow, the S&P and the Nasdaq, and the market has already undergone a 50-percent rally, give or take, so the (expected) positive economic news with regard to the ISM comes in context of an equity market that has to a large degree already priced this in.

S&P 500 futures fell 4.2 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 lost 35 points and Nasdaq 100 futures dropped 6.25 points.

Bank of America Corp is offering to repay part of the U.S. government bailout money, starting with the $20 billion it received in January to help with the acquisition of Merrill Lynch & Co, the Wall Street Journal reported on its website late on Monday. The shares rose 1.2 percent before the bell.

But other financials were weaker, with the Select Sector SPDR Financial ETF and JPMorgan Chase & Co falling 0.4 percent before the bell.

Shares of on-line auction and services company eBay Inc rose 1.5 percent to $22.44 after the New York Times said eBay will sell its online telephony unit Skype to a group of private investors.

The ISM's manufacturing index, due at 10 a.m. along with the housing data, will likely show a reading of 50.5, up from 48.9 in July, according to Reuters' forecasts. That would be the first positive reading since just after the recession began. A reading above 50 points to expansion.

Pending home sales are expected to rise 2.0 percent in July, a slowing from June's 3.6 percent.

(Reporting by Edward Krudy; Editing by Padraic Cassidy)