"It is just simply not useful to get into fantasyland," Rumsfeld said.

The New Yorker magazine reported over the weekend that the Bush administration was considering the use of bunker-busting tactical nuclear weapons against alleged Iranian underground nuclear sites.

"We have I do not know how many various contingency plans in this department," Rumsfeld said at a press conference with General Peter Pace, the chairman of the Joint Chiefs of Staff.

"And the last thing I am going to do is to start telling you or anyone else in the press or the world at what point we refresh a plan or do not refresh a plan and why," he said. "It just is not useful.

"And I have responded with respect to Iran," he added. "We are on a diplomatic track. The president has said exactly what he wants said. And we support the president."

Pace refused to discuss the military's ability to take out deeply buried bunkers with conventional weapons, saying he did not want to give away secrets to the enemy.

But senior US military officials have said publicly that while some conventional weapons can dig through concrete, steel and earth, their effectiveness depends on how deeply buried the target is.

"It depends on how deep and it depends on how the structure is put together," General T. Michael Moseley, the air force chief of staff, told reporters earlier Tuesday.

"There are potentials I would suppose of things so deep and so hardened that it would be hard to get through with anything," he said.

In arguing for funding for the so-called Robust Nuclear Earth-Penetrator feasibility study, Rumsfeld told the Senate a year ago that "the only option we currently have is to use a vastly overpowered nonconventional weapon."

The New Yorker story said the attention to nuclear weapons in the Iran planning raised serious misgivings within the offices of the Joint Chiefs of Staff, and some officers were talking about resigning.

Asked about the story, Moseley said he was not planning to resign and that he had not taken part in any debate or discussion of the kind discussed in the report.

Pace, meanwhile, used the press conference to respond to growing calls for Rumsfeld's resignation by respected retired generals.

The latest was retired lieutenant general Gregory Newbold, the operations director of the Joint Staff through the war in Afghanistan, who called on serving officers to speak up.

"With the encouragement of some still in positions of military leadership, I offer a challenge to those still in uniform: a leader's responsibility is to give voice to those who can't -- or don't have the opportunity to -- speak," Newbold wrote in a column in Time magazine over the weekend.

Rumsfeld said he was not aware of Newbold's criticism in the lead up to the Iraq war. Pace said the general left the Pentagon in September 2002 and did not have personal knowledge of the later phases of the military planning on Iraq.

"We had then, and have now, every opportunity to speak our minds. And if we do not, shame on us, because the opportunity is there. It is elicited from us. And we're expected to," Pace said.

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