(Adds more comments from U.S. officials and analysts)

By Caren Bohan

WASHINGTON, March 28 (Reuters) - The United States insisted on Wednesday it was not escalating tensions with Iran by holding naval exercises in the Gulf and backed British efforts to free 15 sailors and Marines captured last week.

The Bush administration appeared to be adopting a cautious tone on the standoff over the British sailors so as not to disrupt Britain's negotiations with Tehran.

"There is no escalation of tensions on our part," White House spokeswoman Dana Perino said. She said President George W. Bush spoke with British Prime Minister Tony Blair earlier on Wednesday about Iran, among other things, in a previously scheduled, secure video conference.

"The president fully backs Tony Blair and our allies in Britain," Perino said.

Analysts said the effort to keep the U.S. rhetoric subdued was probably the wisest course for now.

"The best way to deal with this is for the United States to stay out of it and allow the British, who have a long history of relations with Iran, to work it out," said Judith Kipper, foreign policy analyst at the Council on Foreign Relations.

U.S. naval exercises in the Gulf have rattled global financial markets, sending oil prices higher and contributing to declines in stock prices. Markets got a jolt late on Tuesday by a rumor -- which proved unfounded -- of a clash between Iran and the U.S. Navy.

Oil investors are worried that a nuclear dispute between Iran and the West, and Tehran's seizure of the British personnel, could escalate and disrupt oil moving through the Strait of Hormuz, which handles about a third of the world's sea-borne oil shipments.

An official with the U.S. government's energy-forecasting agency held out the possibility of using the U.S. Strategic Petroleum Reserve to counter any short-term disruption in oil shipments from the Middle East.

The U.N. Security Council voted to tighten sanctions against Iran over its nuclear program. Iran denies it is seeking nuclear weapons and has called the sanctions illegal.

ACCELERATE WAR GAMES PLAN

For the first time since the U.S.-led invasion of Iraq in 2003, a second U.S. aircraft carrier began exercises in the Gulf.

In Bahrain, a spokesman for the U.S. Fifth Fleet said concern over Iran's recent actions played a role in the decision to accelerate the navy war games, but it was "not the sole determinant."

Perino said the exercises were long planned. She noted that Bush talked of the carrier deployment in a Jan. 10 speech outlining his strategy for Iraq in which he also vowed to "protect American interests in the Middle East."

Perino had no immediate reaction to Iranian statements that they would free a woman among the 15 British sailors held.

The sailors were seized near the waterway separating Iran and Iraq. Tehran said the sailors had strayed into Iranian waters but Britain is adamant they were in Iraqi waters.

State Department spokesman Tom Casey said the sailors must be released "unconditionally."

Pressed on whether the United States would consider swapping Iranians held in Iraq for the British sailors, he said: "That is a hypothetical question and I am not aware that anyone has proposed that."

A State Department official said later that the United States was trying to be very respectful of British efforts to get their sailors released.

"We want to make sure that what we do is supportive of British efforts and does not in any way complicate their mission," he said, adding he was not aware of any British objections to the U.S. naval exercises.

(Additional reporting by Sue Pleming and Tom Doggett)

AlertNet news is provided by