Environment officials are working to clean up a spill of diesel fuel on a northern Saskatchewan lake, the worst of its kind in years.

Early indications are that between 5,000 and 6,000 litres of fuel leaked from a storage tank into a stream leading to Welcome Bay, which is near Wollaston Lake in the northeast part of the province.

People in the village of Wollaston Lake and the Hatchet Lake First Nation have been told to stay out of the water and not to drink directly from Welcome Bay.

Water intakes shut off

Wes Kotyk, a spokesman for the provincial Environment Department, said the communities in the area have shut off their water intake to ensure that there are no contaminants getting into their systems.

Other officials said the community has a day and a half supply of drinking water, assuming rationing takes place.

Shortly after the leak was discovered, a valve on the tank was closed to prevent further spillage, Kotyk said.

The RCMP are investigating to see if someone deliberately did something to cause the tank to leak.

A cleanup crew arrived Tuesday night and went to work immediately to control a plume of diesel that's estimated to measure 300 by 500 metres, Kotyk said.