MADRID (AP) — A small amount of heavy fuel oil has leaked from a bulk carrier ship stranded since colliding Monday with another ship near the Bay of Gibraltar, authorities said Thursday. The Captain of the Gibraltar Port said the leak is “fully under control.”

The Port Authority said a small amount of oil escaped the perimeter of a boom placed to contain any spill. A second barrier is planned to be deployed Thursday to prevent a further expansion of the spill. An aerial photo of the stricken ship shows a small slick on the outside of the boom.

The oil is being collected by two vessels from Gibraltar Port Authority and Spanish Maritime Rescue. The environmental impact of the spill was not immediately clear. Divers sent to the ship have already sealed the source of the leak — two vents in the fuel tanks.

Fabian Picardo, the head of Gibraltar's government, explained to Spanish public broadcaster TVE that the pumping of the fuel remaining in the tanks will be carried out by the ship's insurer.

The extraction is set to last around 50 hours. The ship has stored 215 tons of heavy fuel oil, 250 tons of diesel and 27 tons of lubricating oil, Spanish news agency EFE reports.

The usually busy Gibraltar Port remains closed.