U.S. crude oil stockpiles rose last week amid strong imports, but oil prices did not fall as traders focused on a surprising drop in heating oil inventories.

Crude stocks rose by 1.4 million barrels to 333.8 million barrels in the week ended May 27 as imports rose almost 400,000 barrels per day to 10.7 million bpd, the Energy Information Administration (search) said Thursday.

Analysts in a Reuters poll had expected crude stocks to be unchanged from the previous week.

Crude inventories are now 32.6 million barrels higher than this time last year, indeed, staying near six-year highs.

"This is a very high level of (crude) imports," said Tim Evans, an analyst at IFR Energy Services in New York. "It's impressive that we continue to build crude stocks despite a high refinery operating rate. We have to ask the question once again, where's the shortage?"

Distillate stocks (search), which include diesel fuel and heating oil, rose less than expected by 700,000 barrels to 106.4 million barrels, staying in the lower half of the average range for this time of year.

Heating oil stocks actually fell by 700,000 barrels, but were offset by a larger increase in diesel fuel stocks.

"The distillate build was good, but there was a surprising drop in heating oil, and that is supporting the market," said Phil Flynn, an analyst at Alaron Trading in Chicago.

July oil futures on the New York Mercantile Exchange (search) were up 10 cents at $54.70 a barrel after initially falling on the EIA data.

Refineries operated at 96.2 percent of capacity last week, up 1.6 percentage points from the week before. Analysts in the Reuters survey had predicted an average increase of only 0.3 percentage point.

Gasoline stocks, key at the beginning of the summer driving season, rose more than expected by 1.3 million barrels to 216.7 million barrels.

Gasoline stocks are 12.7 million barrels above this time last year, putting them close to the upper end of the average range for this time of the year.

Total product supplied over the last four-week period has averaged over 20.4 million bpd, or 1 percent more than averaged over the same period last year.

Over the last four weeks, motor gasoline demand has averaged over 9.3 million bpd, or 1.8 percent above the same period last year, while distillate fuel demand has averaged nearly 4.1 million bpd, or 5.2 percent above the same period last year.