The spill, reported to be 20km (12 miles) long, is being blown towards the Argentine capital, Buenos Aires.

Officials say there will be much greater harm to the environment if the fuel reaches land.

No-one was injured in the collision between a Greek and Maltese vessel, which happened on Wednesday.

The incident happened about 20km off the coast of the Uruguayan capital, Montevideo.

The Greek-registered cargo ship Syros leaked the oil after its fuel tank was ripped open in the collision with the Maltese-registered Sea Bird.

Officials are working to dissolve the spill before it reaches the mouth of the Rive Plate near Buenos Aires.

"There's less impact as long as the spill stays on the river, but if the fuel reaches land, the harm is greater," an official at the Uruguayan environment agency told Reuters news agency.

Captain Marcelo Larrobla, from the Uruguayan navy, said the cause of the crash was being investigated.

BBC