The attractiveness of boost phase interceptors lies in their ability to shoot down ascending missiles, whose massive heat signature makes them easier targets, before they release their multiple warheads.

Some serious money is beginning to pour in. The development budget is projected to more than quadruple this October -- from the current 118 million dollars to 511 million, according to congressional officials.

The growth rate will be almost as impressive in subsequent years: from 1.1 billion dollars in 2006 to 2.2 billion in 2009. Flight testing, say defense officials, is slated for fiscal 2010.

But the rocket-like ascent of the program is prompting some on Capitol Hill to wonder whether efforts to counter the missile threat from Iran will push the United States into questionable strategic alliances -- and will add new rationale to the already controversial US military presence in the region.

"It's just that they are throwing huge amounts of money trying to get the technology up and running without thinking clearly about the system they are going to construct," complained Jack Reed, the ranking Democrat of the US Senate Emerging Threats and Capabilities Subcommittee.

"It raises issues of basing it in places like Turkmenistan, Afghanistan, Iraq or the Caspian Sea," the Rhode Island senator told AFP. "And that introduces geopolitical considerations."

While key variables remain unknown, experts agree that if Iran, as expected, produces an intercontinental ballistic missile sometime within the next decade, the United States will not be able to counter it just from ships patrolling the Gulf.

"Discussions are underway with international partners on ways in which they may be able to cooperate," replied a defense official when asked whether the governments of Iraq, Afghanistan and Turkmenistan had already been approached.

The official, who spoke on condition of anonymity, said the Pentagon was focusing on "a mobile, surface-based, multi-use interceptor," whose design will be "based on mature and proven technologies."

To technical experts it means the military is setting its sights on truck-mounted devices with speeds of up to eight kilometers per second while forgoing a 10-kilometer-per-second version, which could weigh up to 80 tonnes and thus require a stationary launch pad.

But in missile defense speed is of the essence, and the choice of slower interceptors, scientists say, will have to be compensated by location.

With the boost phase lasting only about three minutes, of which at least 30 seconds will have to be spent on detecting the launch and producing a firing solution, positioning the interceptors as close to presumed Iranian launch sites as possible will be crucial, experts said.

"Two to four BPI sites would be necessary to fully defend against liquid-fuel ICBMs launched from possible Iranian locations, given commit times between 30 and 90 seconds and interceptor speeds between six and 10 kilometers a second," concluded a Congressional Budget Office report issued in July.

To ensure full coverage of Iranian territory, they will have to be deployed in either in Turkmenistan and the Gulf, eastern Iraq and western Afghanistan, or in all four locations, according to the report.

A study by the American Physical Society issued last year pointed to a floating launch platform in the Caspian Sea, or a base in Azerbaijan, as another viable solution.

The bases will hardly be small. Each of them will likely house three mobile launchers with two interceptors each, noted congressional officials.

The relative proximity of Iran would mean they will be within range of Iranian combat aircraft and missiles and will therefore require their own air defenses, including Patriot batteries.

Defensive perimeter rings will have to be set up to protect the bases against ground assault, the officials pointed out.

The Pentagon declined to comment on these issues. But a defense official underscored the importance of early-stage missile defense, pointing out that all other components of the system, including those being deployed in Alaska this year, were a "back up to the boost phase system."

Senator Reed, meanwhile, believes the Pentagon has not been straightforward with Congress about all the implications of the project.

"It's all just 'Give us a lot of money and trust us,'" he said. "And that's one of the problems, I think."

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