"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 said the test 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.

The aim is to measure the effect of the blast on hard granite structures, he said.

"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, which is scheduled for the first week of June at the Nevada test range.

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