San Diego County beaches near the U.S.-Mexico border remained closed Wednesday as Mexican officials scrambled to halt a sewage spill in Tijuana that has dumped more than 1 million gallons of raw sewage into the ocean since Monday.

Strong southern currents are expected to sweep contaminated waters away from San Diego-area beaches, but health officials closed coastal areas as a precautionary measure, said Steve Smullen, area operations manager for the International Water and Boundary Commission.

Signs warning of sewer contamination are posted from the border to the north end of Imperial Beach.

The sewage is spilling from a pipeline break in Tijuana, about a mile south of the border. Mexican officials have closed beaches in the area.

Its unclear what caused the spill. Environmentalists have blamed aging infrastructure in the past.

Beach closures are common after the Tijuana River swells with runoff from Mexico after heavy rains. The last sewage spill took place in January, 2010, Smullen said.

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San Diego beaches remain closed after Tijuana sewage spill