By William Maclean and David Alexander

DUBAI/WASHINGTON (Reuters) - Washington is signaling its military commitment to its Gulf Arab allies at a time of unfamiliar strain in their decades old partnership.

Syria's civil war and Iran's nuclear program have led to tensions, with Gulf Arab states willing a more assertive U.S. response to bring Iran to heel and force Syrian President Bashar al-Assad from power. Growing U.S. energy independence has further complicated a relationship founded on oil and defense.

Some Gulf Arab oil states had even begun to wonder if the alliance was anything more than the "practical marriage of convenience for a finite period of time" that Henry Kissinger, in a 2007 interview, saw in Washington's ties to Saudi Arabia.

Washington is moving to dismiss such doubts, indicating its military partnership with the ruling dynasties who sit on a third of the world's conventional oil reserves will remain deep-rooted, even if trimmed by budget cuts at home.

"The United States isn't going anywhere. The United States is firmly committed to the security of all our regional partners," said a senior official accompanying U.S. Defense Secretary Chuck Hagel in the Middle East last week. "We understand in a clear-eyed way what the threats are in the region."

Hagel began his week-long trip days after the Pentagon said it was finalizing a $10 billion arms deal that would strengthen the militaries of Saudi Arabia and the United Arab Emirates, as well as that of Israel.

The agreement, more than a year in the making, would result in the sale of 25 F-16 Desert Falcon jets worth nearly $5 billion to the UAE. The UAE and Saudi Arabia also would be allowed to purchase weapons with so-called "stand-off" capabilities that enable them to engage the enemy with precision at a distance.

A few days before the trip, President Barack Obama welcomed to the White House Crown Prince Mohammed bin Zayed of the UAE, the largest U.S. export market in the Middle East.

In a joint statement they reaffirmed a shared commitment to "close defense and security cooperation, including joint training exercises, counterterrorism cooperation and the deployment of interoperable U.S. defense systems".

The deal, and Sheikh Mohammed's warm U.S. welcome, are the latest in a series of signals Washington is sending to the region of its undimmed resolve to support the ruling families, partners in Washington's confrontation with Iran.

OIL AND THE WORLD ECONOMY

The West's energy watchdog, the International Energy Agency, says it expects a continued fall in U.S. oil imports, with North America becoming a net oil exporter by around 2030 and the United States becoming almost self-sufficient in energy by 2035.

Some Gulf Arabs worry that a United States self reliant in oil might show less commitment to safeguarding the Strait of Hormuz, the world's main energy artery through which 40 percent of the world's sea-borne oil exports pass.

Ensuring global energy security is likely to remain an important part of U.S. strategy, said a U.S. official based in the Middle East. While U.S. purchases of Gulf Arab oil may be declining, global dependence is increasing. This fact engages American support, he said.

"Since the health of the U.S. economy is closely tied to the world's, there's every reason for Washington to help protect its allies here," said Les Janka, a former White House and Pentagon official who now heads a business consultancy in Riyadh.

When General Martin Dempsey, chairman of the U.S. Joint Chiefs, told the Center for Strategic and International Studies think tank on March 18 that he hoped America would achieve energy independence, he added: "But I can assure you that at least from a military perspective . . . you will find that the future will be a period of greater commitment."

Pressed to elaborate by UAE envoy Yousef Al Otaiba, Dempsey replied that that commitment should not be measured in terms of carrier battle groups but in terms of improved collaboration.

"We just have to figure out how to help you do more, so that we can do less, but that doesn't mean less well."

Energy is far from the only concern. A U.S. pivot to Asia, perceptions of U.S. economic decline and U.S. military withdrawals, first from Iraq and now from Afghanistan, have blurred Gulf Arabs' security landscape.

Fear of Arab Spring contagion, too, means that Gulf Arab authorities see future threats coming as much from internal sources as from the external ones that were the original premise for Western military support.

Shashank Joshi of Britain's Royal United Services Institution said Gulf Arab officials appeared acutely aware the internal threat they now faced "doesn't quite yoke the Americans in the same way" the external threats once did.

On Syria, Iran and Bahrain, arenas for a region-wide tussle for influence between Sunni and Shi'te powers, Washington's preference for dialogue appears weak to some Gulf Arabs.

"HERE TODAY, GONE TOMORROW"

In Bahrain, home of the U.S. Fifth Fleet and an important Western ally in keeping Hormuz open, a simmering revolt by its Shi'ite Muslim majority has prompted calls by some in Washington for U.S. ships to base elsewhere.

Echoing the view of many Western analysts, Dubai-based Robert Jordan, a former U.S. ambassador to Riyadh, said he was concerned about the message - "we're here today and gone tomorrow" - that a pullout would send. "Perception becomes reality in this part of the world," he said.

Gulf rulers are acutely aware they depend for their security on people who live thousands of miles away, who do not share their religion or lifestyle, and above all who sympathize with the democratic impulse at the origin of the Arab revolts.

In December a summit of the six-nation Gulf Cooperation Council announced plans to set up a unified military command to tighten defense cooperation.

GCC states are increasingly well-armed. Defense spending by GCC states rose about 9 percent to $74 billion last year, estimated Nicole Loeser, Middle East analyst at Forecast International. She predicts it will hit $86 billion in 2017.

Yet Gulf Arab states have faced a host of obstacles to military integration, including a lack of common equipment and their own reliance on bilateral accords with their U.S. ally.

Abdulkhaleq Abdullah, a political scientist in the United Arab Emirates, said confidence in America had dropped because of disagreements with Washington over Syria and dismay over its security failures in Iraq and Afghanistan.

But America remained indispensable. "We live in a very dangerous region, so ... our ties with America remain strong".

(Additional reporting by Phil Stewart in Washington and Mahmoud Habboush in Dubai; editing by Janet McBride)