NEW YORK (Reuters) - Some U.S. refiners have begun reducing crude processing as crude oil costs have risen following U.S sanctions on Venezuela and as gasoline margins crashed to their lowest in nearly a decade, market sources said on Thursday.

Venezuela ships about 500,000 barrels of crude oil to the United States daily, which refiners use to produce gasoline and diesel. But sanctions announced on Monday by Washington, aimed at driving President Nicolas Maduro out of power after his contested re-election last year, are expected to cut off or dramatically reduce those shipments.

Many U.S. refineries, particularly those in the Gulf Coast, are designed to run heavier grades of crude, a good portion of which comes from Venezuela. However, supplies of heavy crude in the Americas are shrinking due to the sanctions on Venezuela, low production in Mexico and bottlenecked pipelines in Canada.

Refiners have been trying to offset this in recent days by buying U.S. domestic heavy and sour crude grades. Those barrels have gotten more expensive, however, because of the higher demand.

U.S. refiners have the capacity to shift the type of crude they process, but light crude commonly produced in Texas tends to refine into additional barrels of gasoline, for which margins have slumped.

U.S. gasoline margins sank to as little as $3.64 a barrel on Thursday, the lowest since November 2009 and the weakest seasonally since at least 2005, as a domestic glut has weighed on pump prices.

The reduction in rates are mostly at Gulf Coast plants but some on the East Coast are also affected, market sources said.

“It’s certainly a combination of both factors,” said one source at a U.S. East Coast refinery where rates are being cut.

The extent of the decline in crude runs was not immediately clear. Top U.S. refiner Valero Energy Corp said it has stopped taking Venezuelan crude and is looking to maximize the amount of light crude barrels it can use.

Running higher amounts of light crude in refinery units designed to break down heavy oil could pose operational difficulties by producing too much gasoline and related fuels, forcing complex refiners to throttle back production, traders and refining consultants told Reuters.

A company spokeswoman declined to comment on crude processing rates.

Refinery maintenance season should help offset some of the impact of shortages in heavy crude supplies, sources said.

Valero, for example, is performing overhauls on the crude distillation and coker units at the St. Charles, Louisiana refinery, one of its two plants that routinely process Venezuelan crude. This minimizes the impact of the sanctions, said Senior Vice President Gary Simmons in a conference call with Wall Street analysts.

One source at a large U.S. Gulf Coast refinery said locking in prices for Canadian heavy crude in advance has helped shield the company from the impact of higher heavy crude costs.