The Port Authority said a small amount of oil escaped a perimeter placed to contain any spill, but the captain of the Gibraltar Port said on Thursday the leak is “fully under control”.
One person had been arrested in connection with the incident.
A second barrier is expected to be set up around the ships to prevent the spill spreading. An aerial photo of the stricken ship showed 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, that has been stranded since the collision on Monday. 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 cargo ship is understood to have “significant damage to the vessel’s starboard side”, including a gash below the waterline measuring approximately 10m by 4m.
The usually busy Gibraltar Port remains closed, while neighboring Algeciras port is fully operational.