ROME — The cruise ship grounded off Tuscany shifted again on its rocky perch Friday, forcing the suspension of diving search operations for the 21 people still missing and raising concerns about the stability of the ship's resting place.

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

The sensors detected that the ship's bow was moving about 15 millimeters an hour and the stern about 7 millimeters an hour, said Nicola Casagli of the University of Florence, who has been called in by Italian authorities to monitor the ship's stability.

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

The sea floor drops off sharply a few meters (yards) from where the ship is resting, and Italy's environment minister has warned it risks sinking altogether. Storms forecast for later Friday have added to concerns.

"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 from his programmed route. The ship then keeled over on its side. Eleven people are confirmed dead.

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.

On Friday, relatives of some of the 21 missing were at Giglio's port getting briefings from rescue teams.

Casagli told Sky TG24 that some movement in the Concordia was only natural given the immense weight of the steel-hulled ship, which is being held in place by two huge rocks at bow and stern.

But the latest movements indicate it isn't stable, he said. "These are small, regular movements that are being monitored because they're going in the same direction," he told Sky.

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. The evacuation was chaotic and the alarm to abandon the ship was sounded after the Concordia had capsized too much to get many life boats down.