Sunderland City Council and the Environment Agency are involved in dealing with the spill in South Hylton.
The Environment Agency said it was working to stop pollution into the Wear Estuary. The councul said no threat to public health had been identified, although peoply may notice a smell of fuel in the area.
The council said: “A tank containing red diesel at the City Council’s South Hylton depot has leaked and fuel has run into a culvert exiting at Pottery Lane, South Hylton.
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"This happened overnight on Monday 8 November and yesterday Tuesday 9 November.
"The council is working with partner agencies on responding to the incident, this includes the containment, clean-up and monitoring.
"No threat to public health has been identified although the public may notice the smell of fuel in the South Hylton riverside area.
"Alongside the public’s wellbeing and the protection of the river’s wildlife, we are continuing to work with local and national partner agencies.
A spokesperson for the Environment Agency said: “We are working closely with Sunderland City Council and other professional partners to manage this incident. Following the deployment of pollution prevention equipment by our officers last night, Sunderland City Council is now leading the response and have employed a specialist contractor to do this.
“We are continuing to assess the situation and we're advising the council and their contractors on the appropriate measures of clean up and containment, but at the moment we do not believe there is any ongoing impact on the environment.