Crews work to contain a fuel mixture that leaked into the Don River overnight. (Kimberly Gale/CBC)

A fuel spill in the Don River near Eglinton Avenue East has been contained, although it will likely take the better part of the day to clean it up, according to Toronto Fire Services.

The leak originated overnight from an underground tank near Eglinton Avenue East and Leslie Street that construction workers unearthed last week north of the river. While they were working, they "ruptured a section of the tank," said Capt. Mike Strapko of Toronto Fire Services.

The tank wasn't leaking at the time, he said, and the construction company was scheduled to have it pumped out Monday. But heavy rains Sunday night caused the tank's contents to flow out through the sewer system and into the Don River, he said.

Hazardous material crews and Toronto fire workers have now contained the spill, Strapko confirmed. Some fire crews sent containment booms at Keating Channel at the foot of the Don River in order to ensure none of the fuel enters Lake Ontario.

"It's always important that we get on it immediately to make sure that it doesn't become a threat to the environment and the crews responded quickly, said Fire Platoon Chief Roy Law.

The name of the construction company that was working around the tank, which is estimated to be up to 50 years old, is not known.

The 19,000-litre tank was filled with a mixture of mineral oil, kerosene and aviation fuel, Strapko said, although it is not known exactly how much fuel entered the river.

Authorities do not know the name of the company that left the tank in the ground, but there used to be an airstrip in the area decades ago, said Strapko.

The Ministry of the Environment, which was not immediately available for comment, will assess the damage to the Don River.