Stock index futures pointed to a lower open on Tuesday as investors worried over a report Greece wants to renegotiate a financial aid package and the prospect of rising interest rates.

Greece's government, concerned that the International Monetary Fund could impose tough conditions in exchange for aid, wants to bypass an IMF financial contribution, senior government sources in Athens told Market News International. A Greek finance ministry source denied the report.

Signs of an improving U.S. economy has driven stocks higher, with the Dow index brushing up against 11,000. At the same time, Treasury debt prices have fallen, with benchmark yields climbing to touch 4 percent for the first time in 10 months, worrying some investors concerned about rising rates.

Many of them will be eyeing the Federal Reserve's minutes from its last policy meeting when they are released Tuesday afternoon.

We had a tremendous jump in the 10-year yield last month, and that continues to hold up, said Peter Jankovskis, co-chief investment officer at OakBrook Investments in Lisle, Illinois. That has implications here in the United States for the fragile real estate market.

S&P 500 futures fell 4.3 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 fell 29 points, and Nasdaq 100 futures lost 6 points.

Shares of Massey Energy Co fell 6.9 percent to $50.74 after an explosion ripped through one of its mines in West Virginia, killing 25 coal miners, with four missing deep underground.

Top U.S. coal miner Peabody Energy Corp raised its offer for Australia's Macarthur Coal Ltd to $3.27 billion, but the new offer is a discount to Macarthur's last trading price. Peabody's shares fell 1.0 percent to $45.56 in premarket trade.

Wells Fargo upgraded the U.S. bank sector to market weight from underweight, pointing to a combination of increasingly positive economic data, greater clarity on asset-quality trends, less fear of pending regulatory changes, and still-reasonable valuation.

Wells Fargo said Bank of America Corp and PNC Financial Inc
were best positioned to benefit.

The Federal Reserve will release minutes from its March 16 policy setting meeting, at which the Federal Open Market Committee reiterated its intention to keep interest rates ultra low for an extended period. The committee will issue the minutes at around 2 p.m. (1800 GMT).

We're going to try to glean something from that because everybody wants to know when the Fed is going to make their move, said Arthur Hogan, chief market analyst at Jefferies & Co in New York.

(Reporting by Edward Krudy; Editing by Padraic Cassidy )