AMSTERDAM (Reuters) - The Port of Rotterdam said on Sunday a clean-up operation was underway after an oil freighter punctured its hull while mooring, releasing a “considerable” amount of heavy fuel oil.

Odfjell, the owner of the freighter Bow Jubail, said in a statement the ship had lost 220 tons of heavy fuel oil in the incident, which took place Saturday afternoon.

Rotterdam Port said in a statement much of that amount had initially been contained with shields, but some escaped and was leaking into various arteries of the expansive harbor’s waterways.

“Experts expect that clean-up work will take days if not weeks,” the port said.

According to the Netherlands’ Sea Creatures Rescue Team, several environmental groups and volunteers were working to save as many as 1,000 birds that were covered in the oil as a result.

Rotterdam said it would hold Odfjell responsible for damages. Odjfell said in a separate statement it is assisting in “mitigating actions.”

“The vessel is insured by the P&I club GARD who has also mobilized resources” for the clean-up effort, the company said in a statement published on its website.

Dutch authorities were investigating the incident, and Odjfell said it is conducting its own internal investigation.