The US Navy has begun its most extensive manoeuvres in the Gulf region since the 2003 invasion of Iraq, but denies sabre-rattling aimed at Iran.
The USS Dwight D Eisenhower is off the coast of Iran
Two aircraft carriers, plus their strike groups, have sailed to the region and have begun war games involving as many as 100 US war planes.
The exercises follow Iran's capture of 15 British navy personnel, and high tension over its nuclear programme.
The US Navy said the exercises were not meant to exert pressure on Iran.
US Navy Commander Kevin Aandahl, based with the US Fifth Fleet in Bahrain, across the Gulf from Iran, said: "What it should be seen as by Iran or anyone else is that it's for regional stability and security.
"These ships are just another demonstration of that. If there's a destabilising effect, it's Iran's behaviour."
Each US strike group incorporates an aircraft carrier, a submarine and four or five frigates or destroyers.
The exercises involve more than 10,000 US personnel on warships and aircraft making simulated attacks on enemy shipping with aircraft and ships, hunting enemy submarines and finding mines, the Associated Press reported.
Meanwhile the two groups are also supporting operations and flying missions in Iraq and Afghanistan.
The aircraft carrier USS Dwight D Eisenhower is in the Gulf, within reach of Iraq, while pilots on the John C Stennis, in the Arabian Sea, are flying sorties over Afghanistan.
"You can't fly missions over Afghanistan from the Gulf because you'd have to fly over Iran," explained Lieutenant-Commander Charlie Brown.
Also in the Arabian Sea is a French carrier group, led by the Charles de Gaulle, which is flying missions to Afghanistan.
France is part of the European troika that has been pressing Iran to halt uranium enrichment - a plea it has rejected.
However, Lt-Cmdr Bertrand Bonneau, chief of press for the French navy, said the deployment had "nothing at all" to do with exerting pressure on Iran.
CARRIER GROUP DEPLOYMENTS
Positions are approximate
USS Dwight D Eisenhower in the Gulf, supporting operations in Iraq
USS John C Stennis in the northern Arabian Sea, flying missions over Afghanistan
French carrier Charles de Gaulle also supporting Nato's Afghan operation from the Arabian Sea