An oil spill is being cleaned up in a waterway connected to the south branch of the Chicago River.

The U.S. Coast Guard said it dispatched response units to Bubbly Creek Thursday night, and officials from the Environmental Protection Agency also were called to the scene.

The spill's source is unknown. It's not clear how much oil has spilled.

Access has been closed to Bubbly Creek, which runs south from a branch of the Chicago River to Pershing Road. Another area of oil was found on the Chicago River's south branch between Ashland Avenue and Pulaski Road, the Coast Guard said.

The EPA is using a containment boom at the entrance to Bubbly Creek. The area is closed to all boats and people, and a safety zone has been set up there.

This safety zone does not affect the Chicago Ship Canal, or the river's south branch.