A tugboat that capsized in the Fraser River overnight just south of Vancouver won't be pulled out of the water until Wednesday, but crews are working to limit the amount of fuel leaking out in the meantime.

The 19-metre tugboat, named the George H. Ledcor, overturned at around 10 p.m. Monday on the north arm of the Fraser, near Deering Island.

There were four people on board when the tugboat sank, but they were rescued by another tugboat crew and taken to safety, according to the Canadian Coast Guard.

The vessel has the capacity to carry 22,000 litres of diesel, but environmental response crews are still working to determine how much has actually escaped into the water.

"Right now we don't know how much fuel was actually on board," the Coast Guard's Kiri Westnedge told CTV News.

Reports that tens of thousands of litres leaked into the water are premature, according to officials.

Booms were placed around the tugboat overnight, and absorbent pads were left in the water to soak up any fuel that spilled.

The boat's owner, Ledcor Grup, has hired the Western Canada Marine Response Corporation to clean up the mess, and the Coast Guard said a dive team went into the river to try and mitigate the leakage.

"The divers have successfully closed or plugged the vents on the overturned tug, so we should see a substantial reduction in the amount of fuel that's leaking from the vessel," said Philip Murdock, superintendent of environmental response for the Coast Guard.

But a sheen has already shown up on the shoreline, prompting Vancouver to close the beach at Fraser River Park pending further notice. Staff were on scene telling dog owners to keep their pets out of the water late Tuesday morning.

Richmond also closed McDonald Beach Park on the other side of the river.

The tugboat's propeller could be seen sticking out of the river in the early morning hours on Tuesday, but the vessel has since been completely submerged. The Coast Guard said it has secured both the tugboat and the barge it was carrying to Sea Island.

But efforts to remove it from the water have been delayed because a crane that was brought in to pull it out was deemed too small. Ledcor Group said the removal is expected to happen on Wednesday instead.

"For safety reasons, an additional, larger crane and barge will be used to lift the tug," a spokesperson said in an email.

A number of agencies are working together to assess the impact of the spill, including B.C.'s Ministry of Environment, Environment and Climate Change Canada, Transport Canada, the Musqueam First Nation and the City of Vancouver.

The province said when it comes to spills off the B.C. coast, the "responsible person or spiller is legally required to clean-up or manage the clean-up of a spill," but the government will take over if no one else is willing or able to do so.

Ledcor Group is an international construction company with headquarters in Vancouver and San Diego.

With files from CTV Vancouver's Nafeesa Karim and Maria Weisgarber