About a month after word first leaked that the White House wanted to develop a pair of new low-yield nuclear warheads, Defense Secretary James Mattis on Tuesday characterized the proposal as a “bargaining chip” to persuade Russia to freeze what the United States says is a nuclear-capable cruise missile program.

In congressional testimony here Tuesday, Mattis told Rep. Rick Larsen (D-Wash.) it was “correct” that the proposed low-yield submarine-launched warheads were a “bargaining chip” to bring Russia to the negotiating table after its alleged violation of an important arms-control treaty that forbids certain nuclear cruise missiles.Jim Mattis: Congress should get back in defense budget 'driver's seat'

But when pressed by Larsen, Mattis would not say whether the Trump administration would cancel its plans for the two new submarine warheads if Russia relented and retired the land-based system the Obama administration said Moscow tested in 2014 in violation of the Intermediate-Range Nuclear Forces Treaty. The U.S. subsequently said Russia had deployed the system.

“I don’t want to say in advance of a negotiation and undercut our negotiators’ position, what we would or would not do,”  Mattis said. “The point I would make is that deterrence is dynamic.”

The Trump administration’s official Nuclear Posture Review, rolled out last week, said DoE’s National Nuclear Security Administration (NNSA) should immediately begin developing a new low-yield nuclear warhead in the 5-kiloton range to fit atop Trident ballistic missiles carried by Ohio-class submarines. The warhead would be made from existing nuclear materials in the NNSA’s stockpile.

The review also directs the Pentagon and Energy Department to study reintroducing a submarine-launched cruise missile into the U.S. arsenal. That kind of missile, last seen during the Cold War, would also be tipped with a low-yield warhead made from existing NNSA parts.

More details will be part of the White House’s fiscal 2019 budget proposal, slated to be published Feb. 12. The budget year begins on Oct. 1.