The Trump administration’s lead nuclear negotiator on Tuesday called on the arms control community to publicly back the White House’s drive to condition an indeterminate extension of the New START treaty with Russia on a verifiable freeze in deployment of all nuclear warheads, even those not covered by the bilateral agreement.

“I think it’s important for the arms control establishment to also step up,” Marshall Billingslea, special presidential envoy for arms control, said in a webcast presentation hosted by the Washington-based Heritage Group non-profit. “Now is the time for the myriad of arms control experts that we have out there to register their voice with the Russians [and say] ‘take the deal.’”

As reported by media earlier this week, Billingslea said Tuesday that the U.S. and Russia had a “gentleman’s agreement” to extend New START “for some period” at “the highest levels of our two governments,” and that the deal needed to “percolate down through their systems so that my counterpart hopefully will be authorized to negotiate.”

That prospect appeared to disintegrate in near-real-time minutes after Billingslea’s remarks.

The U.S. offer was “unacceptable,” Sergei Ryabkov, Russia’s deputy foreign minister told the Moscow-affiliated Sputnik news agency in a story that ran not even an hour after Billingslea made his latest public push for the Trump administration’s deal.

New START limits the U.S. and the Russian Federation to 1,550 deployed strategic warheads on a mixture of 700 intercontinental ballistic missiles, submarine-launched ballistic missiles, and heavy bombers. The parties could extend the treaty, negotiated by the Obama administration, for up to another five years.

Billingslea said the U.S. was willing to extend New START only if Russia agreed to freeze deployments of warheads, including tactical warheads not covered by the treaty, and give the U.S. a means of verifying such a freeze. In return, Billingslea said, the U.S. would do the same.

The Trump administration wants to replace, or at least follow-up, New START with a multilateral nuclear arms control deal that also constrains China’s growing nuclear arsenal, plus Russia’s tactical nuclear weapons — relatively small nukes intended to turn the tide of one military conflict, rather than obliterate an adversary’s ability to wage war or recover from a nuclear strike.

Billingslea also sought to enlist the arms-control community for help with that objective, telling non-government and former government experts to “call on the Chinese to sit down, negotiate and come up with something constructive to say.”

China has said it will not participate in such negotiations with the U.S. and Russia. Billingslea said Tuesday that would constitute a violation of China’s obligation under the U.N. Treaty on the Non-Proliferation of Nuclear Weapons to pursue negotiations that could lead to nuclear disarmament.