By Kristin Roberts

MANAMA, Dec. 8 (Reuters) - Iran poses a threat to the United States and the Middle East despite a U.S. intelligence assessment that Tehran halted its nuclear weapons program in 2003, U.S. Defense Secretary Robert Gates said on Saturday.

In a speech to the Manama Dialogue security conference in Bahrain, the Pentagon chief argued Iran still has the capability to restart its weapons program and continues to enrich uranium, an essential part of atomic weapons development.

He also accused Iran of actively supporting insurgents in Iraq and Afghanistan as well as Islamist groups Hezbollah and Hamas, and that its missile program poses a wider threat throughout the region.

"Everywhere you turn, it is the policy of Iran to foment instability and chaos, no matter the strategic value or cost in the blood of innocents -- Christians, Jews and Muslims alike," Gates said.

"There can be little doubt that their destabilizing foreign policies are a threat to the interests of the United States, to the interests of every country in the Middle East and to the interests of all countries within the range of the ballistic missiles Iran is developing."

Gates also argued that the recent U.S. National Intelligence Estimate on Iran's nuclear program did not rule out Tehran restarting its pursuit of atomic weapons. Iran says its nuclear program has only peaceful civilian aims.

"The Estimate is explicit that Iran is keeping its options open and could re-start its nuclear weapons program at any time -- I would add, if it has not done so already," the former CIA director told the conference.

"Although the Estimate does not say so, there are no impediments to Iran's re-starting its nuclear weapons program -- none, that is, but the international community."

Gates urged Iran's neighbors to cooperate more closely in their defence activities to counter Tehran's policies and specifically consider a joint early warning system to detect missile launches. That, he said, could deter Iran from pursuing development of such weapons.

Iran canceled its appearance at the conference.

Gates' comments follow a visit to U.S. forces in Afghanistan and Iraq, where commanders told him Iran continues to provide support to insurgents.

Iran denies U.S. charges that it has armed, trained and funded Shi'ite militias in Iraq, blaming the violence in Iraq on the U.S.-led invasion to topple Saddam Hussein in 2003.

Gates's remarks also come as the U.S. military assesses Iran's ability to disrupt oil shipments through the Strait of Hormuz, a transit route for around 40 percent of all globally traded oil.

Iran has suggested that its Islamic militia forces would be capable of disrupting strategic Gulf oil shipping routes if it was attacked by the United States.

On Iraq, Gates called progress in the security arena real but fragile. He urged Iraq's neighbours to support the Baghdad government.

"There may be some who, because of past resentments and disagreements, might be cheering for failure. I would respectfully suggest that these sentiments are dangerously shortsighted and self-destructive," Gates said. (Editing by Dominic Evans)

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