"I don't want to sound glib here but it is the first time in Nevada that you'll see a mushroom cloud over Las Vegas since we stopped testing nuclear weapons," said James Tegnelia, head of the Defense Threat Reduction Agency.

Tegnelia's remark at a breakfast with defense reporters set off shock waves of its own here.

"Anytime an administration official starts talking about mushroom clouds and Las Vegas, I want answers," Representative Shelley Berkley, a Democrat from Nevada, said on the floor of the House of Representatives.

She said Tegnelia told her a mushroom cloud would not be seen "over" Las Vegas, only "from" Las Vegas, according to a statement from her office.

DTRA also issued a statement later in the day that said "the experiment" posed no environmental or health hazards, and stressed that it was not a nuclear test.

"All explosives, given the right thermal characteristics, will create a cloud that may resemble a mushroom cloud," the agency said.

"The dust cloud from Divine Strake may reach an attitude of 10,000 feet and is not expected to be visible from Las Vegas," it said.

The agency said no radioactively contaminated soils are in the vicinity of the proposed detonation site, which is over an existing tunnel at the Nevada Test Site.

Tegnelia said the test, which is scheduled for June 2, was part of a US effort to develop weapons capable of destroying deeply buried bunkers housing nuclear, chemical or biological weapons.

"We have several very large penetrators we're developing," he told defense reporters.

"We also have -- are you ready for this -- a 700-tonne explosively formed charge that we're going to be putting in a tunnel in Nevada," he said.

"And that represents to us the largest single explosive that we could imagine doing conventionally to solve that problem," he said.

DTRA said the charge consists of 700 tonnes of commercial ammonium nitrate-fuel oil (ANFO) explosives, which is the equivalent of 593 tonnes of TNT.

"ANFO is a commonly used blasting agent in commercial mining and commercial blasting operations," the agency said. "The purpose is to conduct research into ground motion."

Tegnelia said the aim is to measure the effect of the blast on hard granite structures.

"If you want to model these weapons, you want to know from a modeling point of view what is the ideal best condition you could ever set up in a conventional weapon -- what's the best you can do.

"And this gets at the best point you could get on a curve. So it allows us to predict how effective these kinds of weapons ... would be," he said.

He said the Russians have been notified of the test.

"We're also making sure that Las Vegas understands," Tegnelia said.

DTRA said it did not expect the test to have an adverse impact on the environment or the health of either local residents or participants in the exercise.

"The potential for Divine Strake to create health and safety impacts to the general public is minimized by a combination of the remote location of the NTS, the sparse surrounding population, and a comprehensive program of administrative and design controls," the agency said.

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Defense Threat Reduction Agency