ABOARD THE USS JOHN C. STENNIS — American warplanes screamed off two aircraft carriers yesterday 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.

The Pearl Harbor-based destroyer USS O'Kane is part of the Stennis battle group.

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 said.

Prime Minister Tony Blair warned yesterday that his government could make public evidence that a British navy crew was in Iraqi waters when it was captured by Iran, saying he was prepared to take the standoff to a "different phase" if diplomacy fails to win their release.

Iran said the 15 British sailors and marines were being treated well, but refused to say where they were being held, or rule out the possibility that they could be brought to trial for allegedly entering Iranian waters.

"I hope we manage to get them to realize they have to release them," Blair said in an interview with GMTV. "If not, then this will move into a different phase."

Oil prices shot nearly 8 percent in a matter of minutes in after-hours trading yesterday, 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's 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.

KEEPING 'VITAL LINK' OPEN

The exercises began four days after Iranian forces detained the 15 Britons for allegedly being in Iranian territorial waters near the northern end of the Gulf. U.S. and British officials insist the team was properly searching cargo vessels inside Iraqi waters.

F/A-18 fighter jets roared off the Stennis' flight deck all day, mounting a dozen rapid-fire training sorties against imaginary enemy ships and aircraft. A second task force with the carrier USS Dwight D. Eisenhower also took part in the drills.

"These maneuvers demonstrate our flexibility and capability to respond to threats to maritime security," said Navy Lt. John Perkins, 32, of Louisville, Ky., as the Stennis cruised about 80 miles off the United Arab Emirates after entering the Persian Gulf overnight.

"They're showing we can keep the maritime environment safe and the vital link to the global economy open."

At the headquarters of the Navy's 5th Fleet in Bahrain, Cmdr. Kevin Aandahl said the maneuvers would last several days. He said U.S. warships would stay out of Iran's territorial waters, which extend 12 miles off the Iranian coast.

None of America's naval coalition partners in the region joined the maneuvers.

10,000 U.S. PERSONNEL

A French naval strike group, led by the aircraft carrier Charles de Gaulle, was operating just outside the Gulf in the Arabian Sea. But the French ships were supporting NATO forces in Afghanistan and not taking part in the U.S. maneuvers, Aandahl said.

At the Pentagon, spokesman Bryan Whitman said the Navy routinely conducts exercises when its forces are deployed near each other.

"The exercise should reassure our friends and allies of our commitment to security and stability in the region," Whitman said. "We are not interested in confrontation in the Gulf."

The war games involve more than 10,000 U.S. personnel mounting simulated attacks on enemy aircraft and ships, while hunting submarines and looking for mines.

"What it should be seen as by Iran or anyone else is that it's for regional stability and security," Aandahl said. "These ships are just another demonstration of that. If there's a destabilizing effect, it's Iran's behavior."

The U.S. drills were the latest in a series of competing American and Iranian war games. Iran conducted naval maneuvers in November and April; in October the U.S. Navy led a training exercise aimed at blocking nuclear smuggling.