Defense Secretary Jim Mattis told Congress Wednesday he has an open mind about possibly scaling back some nuclear systems, as long as deterrence is not sacrificed, as he faces a more than $1 trillion bill to rebuild America's arsenal over the next three decades.

"We're looking at each leg of the triad and we're looking at each weapon inside each leg," Mattis testified before a Senate subcommittee reviewing the Pentagon budget request for fiscal 2018. "What I'm looking for is a deterrent that will be most compelling to make certain these weapons are never used."

Mattis is referring to the Pentagon's Nuclear Posture Review, a top-to-bottom assessment of U.S. nuclear capabilities and strategy, including the Cold War era "triad" of bombers, submarines and ground missiles designed to ensure the U.S. could counterattack after a first strike.

Mattis' comments came under questioning from California Democrat Sen. Dianne Feinstein, who has been waging a lonely fight against one nuclear weapons system in particular, the Long Range Stand-Off, or LRSO, air-launched nuclear cruise missile.

"I do not see it as an effective deterrent weapon," Feinstein said. "I see it as Russia taking action to counter it and with the cost and the fact that we've got new ballistic missile submarines, new bombers, new intercontinental ballistic missiles and new warheads, I wonder why we need to develop this specific weapon? The cost is going to be inordinate."

Mattis replied that his initial take is that the bomber leg of the triad will need the stand-off capability of an air-launched cruise missile to get past increasingly sophisticated enemy air defenses, but said he's still studying the system.

"I've not yet completed my own review," Mattis told Feinstein. "I would say though that we have got to make certain the bombers can get through if they're to be a valid deterrent from these weapons ever being used."

Feinstein seemed to be encouraged when Mattis said he would be consulting with former Defense Secretary William Perry, who has advocated eliminating one leg of the triad by phasing out the land-based intercontinental ballistic missiles.

Perry is also strongly opposed to developing new nuclear cruise missiles, which he says are "uniquely destabilizing" weapons, because an adversary cannot tell a conventional missile from a nuclear-armed version, risking miscalculation in a crisis.

"I register loud and clear the potential destabilizing view that some people see this weapon bringing and I'm taking that on board," Mattis said. "But I've got to do more study."

Feinstein said she has had extensive discussions with people in the military and has concerns that the LSRO may in fact represent a new generation of nuclear missiles not just an upgrade of older air-launched missiles.

"It's got features which concern me greatly," she said. "I'm not sure for the cost that we'll end up with a practical deterrent."

Feinstein requested a private meeting to discuss classified details of the project, for which the Trump administration has requested $500 million in development costs for next fiscal year.

Mattis said he'd be happy to meet with the senator, who sits on both the Appropriations and Armed Services Committees.

"I'd be eager to get that perspective, ma'am, because I want to give the right kind of guidance to this posture review," Mattis said.