Oil from an unknown source has been polluting a Norfolk river for the past two weeks, the Environment Agency said.
The substance was spotted in the River Yare at Colney, near Norwich, on 22 December, and has been "intermittently" leaking from a drain near Chancellors Drive since.
The EA said it was taking measures to contain the pollution while it tried to find the source.
People are being advised to keep themselves and pets out of the river.
A spokeswoman for the EA said: "A team of specialists were on site yesterday to remove some of the oil caught around the drain and to contain as much of the pollution as possible until the source can be found."
Oil has been spotted on the river past the University of East Anglia Broad, and downstream as far as Lakenham in Norwich.
The EA said there was no evidence the UEA Broad itself had been affected, but Dr Ian Barr, who lives nearby and works at the university, said there had been "surface contamination" and the smell was "still evident".
"One of the saddest things I've seen over Christmas is a little grebe, one of the most beautiful birds in the UK, on the river diving through diesel oil catching contaminated fish," he said.
"I've seen them (Environment Agency officers) three times. But it's all a little slow.
"Today they said 'our visit this morning showed that nothing further came through the outfall overnight', but yet the river today was the same."
The EA said it was awaiting test results to determine what type of oil was leaking into the river.