Eighty percent of the diesel from a damaged and partly sunk bulk carrier ship has been removed, Gibraltar authorities said, though 180 tonnes of heavy fuel oils remains onboard.

The ship was ordered beached after colliding with another vessel on Monday in the bay of the British territory, located at the western entrance to the Mediterranean Sea.

A government spokesman said there had been no further seepage of heavy fuel since Thursday, with the situation under control.

The spokesman, who was not authorised to be named publicly, said the ship had been carrying 250 tonnes of diesel and still had 183 tonnes of heavy fuel oil and 27 tonnes of lubricant oil in its tanks.

The heavy fuel oil is potentially more damaging to the environment and more difficult to extract, raising concerns in Spain and Gibraltar for marine life and tourism in the area.

Salvage efforts may be announced later on Friday, the spokesman said.

An aerial photo of the stricken ship showed a slick on the outside of the booms placed to contain it.

Officials in the nearby Spanish town of La Linea said they would look at legal action if the oil reaches the coast.

The environmental impact or the quantity of oil spilled was not immediately clear.

Divers sent to the ship have already sealed the source of the leak.

The usually busy port of Gibraltar remains closed, but the neighbouring Algeciras port in Spain is fully operational.

The 178-metre Tuvalu-registered OS 35 was carrying a cargo of steel bars.

The LNG carrier with which it collided sustained little damage.

No one was hurt in the crash.

The captain of the damaged ship has been detained for allegedly not obeying Gibraltar port orders initially after the collision.