The Coast Guard is overseeing the cleanup of an oil spill that occurred Tuesday afternoon on the Piscataqua River, which forms the boundary between Maine and New Hampshire.

Enough oily water was discharged from the Ferdinand R. Hassler, a National Oceanic and Atmospheric Administration vessel, to form an oil sheen the size of a football field, said Lt. Scott McCann, a Coast Guard spokesman.

The spill occurred while the vessel was moored at New Castle, New Hampshire. McCann said the Coast Guard is still trying to determine how much oil spilled.

“We regret the incident and are working closely with the Coast Guard and other responders to minimize any environmental impacts,” David L. Hall, a spokesman for NOAA, said in a press release. “We are investigating the incident to understand the cause with an aim to prevent it from happening again. Any amount spilled is too much and we apologize to anyone who may have been affected.”

The hull of one vessel was covered with oil from the spill. There were no reports of pollution on the shore or wildlife being harmed. The cleanup will continue Wednesday.

The river cleanup is being done by Enpro Services of Maine.