The Coast Guard said in a news release that Friday morning's spill began when a Panamanian-flagged vessel Dubai Star was conducting bunkering operations with a fuel barge.
During the operations, an undetermined amount of fuel oil went into the water at Anchorage Nine in San Francisco Bay. As a precaution, a 100-yard security zone was set up around the spill site.
Coast Guard Petty Officer Erik Swanson told the San Francisco Chronicle that following the spill, a one-mile sheen could be seen on the bay near the two vessels.
Nathan Ballard, a spokesman for San Francisco Mayor Gavin Newsom, said the Coast Guard estimates the spill could involve as little as five gallons of oil or less.
The Chronicle said the spill appears much larger than just a few gallons based on aerial footage.
It remains unknown what caused the spill and it was not reported whether the spill has been completely contained.