WASHINGTON - Iran could make enough highly enriched uranium for a nuclear bomb in one year but would most likely not be able to field a usable weapon for 3-5 years, top US military officials said Wednesday.

General James Cartwright, vice chairman of the US Joint Chiefs of Staff, also cautioned often impatient lawmakers that a limited US military strike was not likely to be "decisive" in halting Tehran's suspect atomic program.

Cartwright agreed with Democratic Senator Jack Reed that US-sought sanctions on Iran were not "a magic wand" and told the Senate Armed Services Committee: "Military activity alone is not likely to be decisive either."

Pressed by lawmakers, Cartwright said it would take the Islamic republic one year to make enough weapons-grade uranium and 3-5 more years to field such a weapon if and when it made the decision to pursue such a plan.

He warned he was making "an historical estimate" and stressed "I can't tell you what problems they will encounter" but "experience says that it's going to take maybe 3-5 years" for Tehran "to have a deliverable weapon that is usable."

Iran denies Western charges that its atomic program hides a covert quest for nuclear weapons, but has defied UN demands that it freeze its uranium enrichment process, drawing several rounds of international sanctions.

Cartwright and Lieutenant General Ronald Burgess, the head of the US Defense Intelligence Agency, declined to say whether the US spy community had revised its 2007 finding that Iran's nuclear weapons program is dormant.

Burgess told the committee that a new US National Intelligence Estimate was in the works but that "we do not have insight that the regime has made the decision to move in that direction."

US officials say Iran could act on separate tracks to build up its ability to make a nuclear weapon and the missile technology to deliver it, making it hard to determine how close Tehran is to having an atomic arsenal.

"There are many pieces to this puzzle," US Undersecretary of Defense for Policy Michele Flournoy told the lawmakers. "I think there's always a question of what you don't know."

Flournoy repeated that "all options are on the table" -- code for the possible use of military force -- but underlined: "At this moment in time, we believe that there are other options that need to be pursued in their fullest."

To that end, the United States is pursuing a new round of UN sanctions on Iran with "urgency," and believes that Russia and China, long reluctant to back such measures, are "likely" to sign on, a top US diplomat told the hearing.

"I think it is likely that we will be able to produce a Security Council resolution," said Undersecretary of State for Political Affairs William Burns. "I hope very much in weeks. We're going to work very, very hard."

He was speaking as the six major powers resumed closed-door talks in New York Wednesday seeking to hammer out a new sanctions package with a US draft resolution on the table, a diplomat familiar with the talks said.

But asked whether Moscow and Beijing would agree to cut off Iran's imports of gasoline and other refined petroleum products, as envisioned by the US Congress, Burns replied: "That's going to be very difficult to achieve."

Republican Senator John McCain expressed frustration at repeated, unfulfilled warnings of coming US sanctions, saying: "We keep pointing the gun. We haven't pulled a single trigger yet, and it's about time that we did."

In his testimony, Burgess said Iran had a "largely defensive" military posture and was "unlikely to initiate a conflict intentionally or launch a preemptive attack" but could temporarily close off the nearby Straits of Hormuz -- a key shipping route for Middle Eastern oil.

And "Iran has gone to great lengths to protect its nuclear infrastructure by locating facilities in buried, hardened facilities. It also seeks to protect them by acquiring sophisticated air defense systems," he said.

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