Twelve passengers on a cruise ship moored in Gibraltar were injured Tuesday after a towering dockside tank holding used oil and water exploded as workers were welding it, sending plumes of black smoke into the air and shrouding the British outpost's famed "Rock" from view.

A Spaniard who was welding atop the tank suffered life-threatening burns and was scheduled to be transferred to a burn unit at a hospital in the Spanish city of Seville, Gibraltar's government said in a statement. A second Spaniard suffered less serious injuries.

The passengers aboard the "Independence of the Seas" cruise ship operated by Royal Caribbean suffered injuries ranging from minor burns to a fractured shoulder, the statement said.

Gibraltar's government said the ship sailed away from the burning tank immediately after the blast, and Royal Caribbean said in statement that it immediately moved "a safe distance away from the dock" and then headed toward its next scheduled stop, the French Mediterranean destination of Cannes.

The company characterized the passengers' injuries as minor and said all received medical treatment aboard the cruise ship.

Two guests who were onshore when the explosion happened were not hurt, Miami-based Royal Caribbean Cruises Ltd said. Spokeswoman Cynthia Martinez said she had no information on where the injured passengers were on the ship when they were injured.

Heavy smoke billowed into the air for hours after the blast happened while the workers were welding atop the tank, and efforts to douse the afternoon blaze suffered a setback after a second tank next to the burning one caught fire in the evening, the statement said.

Firefighters who were trying to put out the fire from land were forced to withdraw, and Gibraltar's government said it was using tug boats to spray water on the raging blaze from the sea. Residents living near the burning tanks were not forced to evacuate, but were advised to close the windows of their homes.

An investigation was under way to determine the cause of the blast, Gibaltar's government said.

Royal Caribbean said the ship was on a 14-night cruise that left Southampton, England, last Saturday.

Gibraltar is a popular tourist destination, and the Rock lies at the eastern end of the Strait of Gibraltar, a strategic waterway that links the Atlantic and the Mediterranean and separates Europe from Africa.

An Anglo-Dutch fleet captured Gibraltar in 1704. Spain ceded Gibraltar to Britain in 1713 but never relinquished its claim to it. The outpost has about 30,000 residents.

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Associated Press writer Jorge Sainz contributed from Madrid.