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LONDON, Aug 1 (Reuters) - The U.S. Navy, scaling back its force in the Gulf, said on Wednesday it had sent a fresh aircraft carrier to the region to replace two carriers deployed there since early this year amid tension with Iran.

It said the USS Enterprise Carrier Strike Group, which includes accompanying destroyers and a submarine, was in the Red Sea and heading for the Gulf and the wider Gulf region.

Lt. John Gay of the U.S. Navy's Fifth Fleet headquartered in Bahrain said the two other carrier groups, headed by the USS Stennis and the USS Nimitz, had moved out of the area in July and were now in the Pacific.

The United States has been flexing its muscles in a standoff with Iran over Tehran's nuclear program. Washington also accuses Iran of meddling in Iraq and of fostering militant Islamic groups throughout the Middle East.

But the two old foes have begun tentative talks aimed initially at easing the conflict in Iraq and last month set up a security committee between officials of the three countries.

The Navy said in a statement that the Enterprise would conduct security operations and provide air support for ground forces in Afghanistan and Iraq.

Gay said the Stennis and Nimitz were unlikely to return to the Gulf although he could not rule it out.

"World events are going to drive that -- if there is a significant change in the security and stability in the region we would re-evaluate the number of ships we have in the area," he said.

The United States sent a second carrier to the Gulf at the start of the year. U.S. officials said the move was designed to reassure U.S. allies concerned about Iran's increasing influence in the region.

The United States and its allies say Iran is secretly trying to build a nuclear bomb and are pressuring Tehran to suspend uranium enrichment activities. Tehran insists it is working on developing nuclear energy.

Tensions in the region are a central issue in a tour of the Middle East now under way by U.S. Secretary of State Condoleezza Rice and Defence Secretary Robert Gates.

They visited Saudi Arabia in the past two days partly to discuss a 10-year arms deal for the U.S. ally which is expected to be worth at least $20 billion and is likely to be expanded to include deals with other Gulf states.

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