UPDATE 1-Exxon cleanup of Arkansas oil spill continues, pipeline still shut

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HOUSTON, April 1 (Reuters) - An Exxon Mobil Corp crude oil pipeline that ruptured and spewed thousands of barrels of oil into an Arkansas subdivision remained shut on Monday as cleanup efforts continued with no word on when it would be fixed and restarted.

The company had yet to excavate the area around the Pegasus pipeline breach, a critical step in assessing damage and determining how and why it ruptured.

Exxon spokesman Charles Engelmann declined to speculate on when excavation would start. He said the company's focus remains on cleanup.

The U.S. Environmental Protection Agency, the U.S. Transportation Department's Pipeline and Hazardous Materials Safety Administration (PHMSA), as well as state and local responders, were on the scene as well.

Exxon also did not yet have a specific figure of how much oil was released when the 20-inch line ruptured on Friday. The company said on Sunday that 12,000 barrels of oil and water had been recovered.

Engelmann said the ruptured section of the pipeline was installed in the late 1940s, but had no information on when it last underwent maintenance.

To prevent and track corrosion buildup, pipelines are periodically "pigged," or cleaned with a device that moves through the line to remove buildup of hydrocarbons, dirt, and other substances. Often the device is outfitted with sensors that point out areas of corrosion or wear and tear that need repair.

PHMSA said in a recent report that more than half of the nation's pipelines were built in the 1950s and 1960s in response to higher energy demand after World War II. Some, like Pegasus, were built earlier.

(Reporting By Kristen Hays; Editing by Gerald E. McCormick and David Gregorio)