LISBON, March 12 (Xinhua) -- Renewed attempts began on Monday to free a Spanish cargo ship that has been stuck in a sandbank outside the port of Lisbon for the best part of a week, local media reported.

The boat, a 118 meter cargo ship, ran aground in the early hours of last Tuesday. After leaving the Lisbon docks bound for Casablanca, Morocco, the ship, which sails under the Spanish flag, suffered a power failure.

The skipper attempted to anchor, but fierce waves pushed the boat towards Bugio, a small island with a lighthouse in the River Tagus estuary. The ship, called Betanzos, lodged into a sandbank and has remained there ever since.

Betanzos is carrying 130 tonnes of petrol and 20 tonnes of oil residues. A spillage would, therefore, be catastrophic from an environmental perspective.

However, the Lisbon Maritime Authority has said the risk of spillage is minimal. There is no structural damage to the vessel and the crew took care to move tanks containing the fuel to the stern, the ship's most robust and secure part.

The ten crew members were air lifted out of the boat on Thursday as a precautionary measure due to extreme weather conditions. High winds and huge waves have buffeted Lisbon over the past week.

Four of the ten, plus six rescue specialists, were returned to the ship by helicopter on Monday morning. This team will study the ship's engineering features and general state of repair. Specifically they will try to establish the ship's strongest point with a view to attaching a towing cable.

There have already been three failed attempts to set the ship upright. For the next attempt, a giant tugboat has been brought up from Gibraltar.

A high tide is required to attempt a salvage operation. The next high-tide in the Tagus estuary is due around midnight Monday night and has been described as a good window of opportunity by the maritime authorities.