ROME » The cruise ship grounded off Tuscany shifted again on its rocky perch, forcing the supension Friday of search and rescue operations for the 21 people still missing.

It was not clear if the movements registered overnight by onboard sensors were just vibrations as the Costa Concordia settles on the rocks off the Tuscan island of Giglio or if the massive ocean liner is slipping off the reef.

The ship’s movements are being carefully monitored since any significant shift could be dangerous for divers trying to locate those missing after the Concordia ran aground Jan. 13. An additional fear is that any significant movement could damage tanks holding a half-million gallons of fuel oil.

"The ship is not in safe enough conditions for rescue operations to continue," said Coast Guard spokesman Cmdr. Cosimo Nicastro.

The $450 million Costa Concordia was carrying more than 4,200 passengers and crew when it slammed into well-marked rocks off the island of Giglio after the captain made an unauthorized diversion Friday from his programmed route. The ship then keeled over on its side.

Capt. Francesco Schettino, who was jailed after he left the ship before everyone was safely evacuated, is under house arrest, facing possible charges of manslaughter, causing a shipwreck and abandoning his ship.

Eleven people have been confirmed dead and 21 others are still missing.

Late Thursday, Costa-owner Carnival Corp. announced it was conducting a comprehensive audit of all 10 of its cruise lines to review safety and emergency response procedures in the wake of the Costa disaster.

In addition, the Miami-based company, the world’s largest cruise operator, said it was conducting an outside review of the Concordia grounding itself.