U.S. and Japanese officials are investigating an oil leak Thursday that flowed from Camp Zama into a nearby stream.

“We’re finding out where the compound was coming from. Definitely there was something, a petroleum substance, going out of our storm drain,” Ed Roper, a U.S. Army Garrison Japan spokesman, said Friday.

“We stopped the flow of the material. We’re looking at causes. We don’t know how much yet, but the flow going outside the fence line has been contained.”

Sagamihara city officials said about 600 gallons were soaked up in an oil-absorbent cloth.

An area resident called Sagamihara city on Thursday morning to report a petroleum odor and an oily substance floating in the Hato River, a city official said.

The city notified the base around 10 a.m., Roper said.

Base environmental officials and the fire department responded, quickly placing booms around the material to contain it, Roper said.

The substance was being tested Friday, Roper said.

Sagamihara and Zama cities have asked the base to identify the cause of the leak quickly and to allow Japanese inspectors access to the source.

An environmental preservation official from Sagamihara city said the damage from the leak wasn’t serious.

“At the moment, we believe that there are no obvious effects directly on health and to fish in the river,” the official said.

The waterway is a storm drainage channel leading off base, and Roper said Thursday’s rain may be partly to blame for the incident.

“We’re waiting for a report right now,” he said.