The US has a proven and enduring commitment to Middle East security, backed by diplomatic engagement and a fierce array of warplanes, ships, tanks, artillery and 35,000 troops, Pentagon chief Chuck Hagel told Gulf leaders on Saturday.

The US defence secretary, speaking at a regional security forum, acknowledged Gulf leaders' concerns about the direction of US policy in the Middle East, especially negotiations on Iran's nuclear program. But he said the USemphasis on diplomacy should not be misinterpreted.

"We know diplomacy cannot operate in a vacuum," Hagel said. "Our success will continue to hinge on America's military power, and the credibility of our assurances to our allies and partners in the Middle East."

Hagel's remarks to the Manama Dialogue security forum in Bahrain came at a time of heightened tension in the relationship between Washington and its longtime Gulf partners.

Leaders in the region are worried the US will lose focus on the Middle East as it strategically rebalances to Asia. They also are frustrated by the US response to the Arab Spring protests and the Syrian civil war.

The United States has been cautious about backing rebels fighting to oust Syrian President Bashar al Assad, citing the absence of a unified opposition.

Hagel laid out the US stance on Syria at the Manama Dialogue, saying that while the US would continue to provide aide to Syrian refugees and the neighbouring countries of Jordan and Turkey, the rise of violent extremism in Syria had to be addressed.

"We will continue to work with partners throughout the region to help bring about a political settlement to end this conflict," Hagel said. But he called for efforts to ensure that aid for the opposition "does not fall into the wrong hands."

Hagel said the interim agreement the six powers reached with Tehran over its disputed nuclear enrichment program had "not diminished our focus on the challenges posed by Iran," which has long vied with the Arabs for regional dominance.

"Iran has been a profoundly destabilizing influence, and a nuclear-armed Iran would pose an unacceptable threat to regional and global stability," the US defence chief said.

He said the accord "bought time for meaningful negotiation, not for deception" and that US diplomacy would ultimately be backed up by the military commitments and co-operation it has with regional partners.

"As America emerges from a long period of war, it will not shirk its responsibilities," Hagel said, citing decades of US involvement in the Middle East. "America's commitment to this region is proven. And it is enduring."

To underscore the scope of the US security commitment to the region, Hagel outlined the array of American military forces in the area, including more than 35,000 military personnel "in and immediately around the Gulf." Included in that figure were 10,000 Army soldiers with tanks, artillery and helicopters.

He said the US has deployed its most advanced fighter aircraft in the region, including the radar-evading F-22.

More than 40 Navy vessels patrol the waters nearby, including an aircraft carrier and its supporting warships, Hagel said. US Navy ships have steamed through the narrow Strait of Hormuz at the mouth of the gulf some 50 times in the past six months in the name of ensuring freedom of navigation.

Hagel also cited the United States military facilities in the region, including Fifth Fleet headquarters in Bahrain, where the Pentagon has a $580 million expansion program under way, and the combined air operations center in Qatar.

The Pentagon chief proposed new steps to improve security co-operation, including regular discussions of missile defence with the region's air chiefs and allowing the six-nation GCC to purchase US defense systems as an organization to encourage regional co-operation.-Reuters