A port statement said the spill occurred during a "bunkering operation" -- the process of filling a ship with fuel -- in the Deurganck Dock and was "causing significant disruption" to shipping.

Deurganck is one of the port's most important container docks, used by some of the largest ships in the world to load and unload goods.

"Although the source has been stopped and the directly involved seagoing vessels have been contained for active clean-up operations, the pollution spread further towards the Scheldt (river) overnight," the port said.

The Scheldt is "currently fully closed to traffic" in the section near the Deurganck Dock, with a port spokesman telling AFP traffic was disrupted "certainly for today, possibly for several days".

The tidal river estuary is the main maritime access route to Antwerp port, along with several narrower canals.

"Emergency services are fully mobilised and are focusing on containment and clean-up operations," the port statement said.

Local media reported that the spill occurred during the refuelling of the container ship MSC Denmark VI.

The ship's operator, MSC, confirmed the vessel was involved in the incident but declined to provide additional information.

"Our priority is safety," a spokeswoman for the company's Belgian office told AFP, adding that they were focused on safeguarding "the crew, the terminal, the nature".

Citing a "risk of impact on nearby nature areas", the port said it was "doing everything possible to minimise both operational and ecological damage".

"We regret the impact of this incident on our port and the vulnerable nature areas along the Scheldt," the statement said.

Larger than 22,000 football fields, the Antwerp port is a key gateway for goods coming into Europe, second only to Rotterdam in the Netherlands.

Around 267 million tonnes of goods transit each year through the port on Belgium's north coast, used annually by around 20,000 seagoing vessels and 50,000 inland vessels, according to its website.