ABOARD THE USS JOHN C. STENNIS – American warplanes screamed off two aircraft carriers Tuesday as the U.S. Navy staged its largest show of force in the Persian Gulf since the 2003 invasion of Iraq, launching a mammoth exercise meant as a message to the Iranians.

The maneuvers with 15 warships and more than 100 aircraft were sure to heighten tensions with Iran, which has frequently condemned the U.S. military presence off its coast and is in a face-off with the West over its nuclear program and its capture of a British naval team.

While they would not say when the war games were planned, U.S. commanders insisted the exercises were not a direct response to Friday’s seizure of the 15 British sailors and marines. But they also made clear that the flexing of the Navy’s military might was intended as a warning.

“If there is strong presence, then it sends a clear message that you better be careful about trying to intimidate others,” said Capt. Bradley Johanson, commander of the Stennis.

“Iran has adopted a very escalatory posture with the things that they have done,” he added.

Meanwhile, oil prices shot nearly 8 percent in a matter of minutes in after-hours trading Tuesday, topping $68, as rumors spread that Iranians had fired at a U.S. ship in the Persian Gulf and that Britain had taken action to free the captives.

But both the U.S. military and the British government denied the rumors.

Lt. Cmdr. Charlie Brown of the U.S. Navy 5th fleet told The Associated Press that all ships in the Gulf had been checked and the rumors were untrue. Britain’s Foreign office said its military had not taken any action.

The exercises began four days after Iranian forces detained the 15 Britons.

James Calderwood reported from aboard the Stennis and Jim Krane from Dubai.