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NEW YORK, July 29 (Reuters) - The U.S. Treasury sold $39 billion in five-year debt on Wednesday in an auction that drew poor demand, raising worries over the cost of financing the government’s burgeoning budget deficit.

Demand overall was below average, measured by the bid-to-cover ratio of 1.92, the weakest in almost a year.

In a further sign of weakness, yields at the auction were well above expectations, known as a “tail” by market participants.

A key proxy for foreign interest, the indirect bidder category, was slightly above the average of auctions over the past year at 36.6 percent but far below the most recent sale.

“It was just a horrendous result,” William O’Donnell, head of U.S. Treasury strategy at RBS Securities in Greenwich Connecticut, said about the auction.

“It was the weakest bid-to-cover since September 2008, and by my numbers it was the biggest tail since February 1993. It was just a very, very weak result.”

The tail indicates that dealers drove an unexpectedly hard bargain to raise yields, and lower prices, to buy the bonds, which spooked the bond market.

Five-year notes US5YT=RR fell further, last trading down 10/32, with the yield rising to a four week high around 2.66 percent.

Benchmark 10-year notes US10YT=RR surrendered their gains for the day and dropped into negative territory after the sale. They were last trading down 3/32, yielding 3.71 percent versus 3.69 percent at Tuesday's close.

The five-year sale is part of this week’s record $115 billion in coupon securities being auctioned.

With the government set to issue $2 trillion in new bonds this year to finance economic and financial rescues, investors have been watching for any signs of waning demand for U.S. debt, particularly among foreigners.

Treasury auctions have come under particularly close scrutiny since investors began to question the longevity of the United States’ prized AAA credit rating back in May.