WASHINGTON (Reuters) - U.S. lawmakers of both parties are pressuring the White House to extend the last remaining restraints on U.S. and Russian nuclear weapons deployments by demanding intelligence assessments on the costs of allowing the New START treaty to lapse.

U.S. President Donald Trump speaks during a meeting with Guatemala's President Jimmy Morales in the Oval Office of the White House in Washington, U.S., December 17, 2019. REUTERS/Kevin Lamarque

The demands, contained in three bills that may be harmonized this week, reflect doubts about whether the Trump administration has done sufficient analytical work on how China and Russia may respond to the 2010 treaty’s expiration in February 2021.

New START restricted the United States and Russia to deploying no more than 1,550 nuclear warheads, the lowest level in decades, and limited the land- and submarine-based missiles and bombers that deliver them.

It can be renewed for up to five years if both sides agree. Moscow has offered to immediately extend the treaty. Washington still is considering the issue.

U.S. President Donald Trump and his aides have argued that New START does not cover all Russian nuclear weapons and said they want to bring China, which they increasingly view as the primary, long-term threat, into a wider arms control framework.

Some lawmakers and arms control experts view the proposal as a “poison pill” to kill New START, ending restraints on U.S. strategic nuclear weapons deployments, because China rejects the idea.

In May, Trump announced that he and Russian President Vladimir Putin discussed a possible new accord limiting nuclear arms that eventually could include China.

Three days later China, estimated to have only about 300 nuclear weapons, dismissed the idea of participating in trilateral nuclear arms reduction talks.

China’s arsenal is dwarfed by those of the United States and Russia. Both are estimated to have over 6,000 deployed, stockpiled or retired (and awaiting dismantlement) nuclear warheads, according to the Federation of American Scientists.

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Lawmakers, congressional aides and former officials say they are unaware of the administration conducting any formal intelligence estimates of the implications of New START’s expiration either before or after Trump unveiled the idea.

Nor are they aware of extensive inter-agency deliberations on devising a negotiating stance with China, or even whether any negotiations with China have occurred.

“What we don’t want to see is ... China used as an excuse to blow up the existing, or potential extension of an agreement with Russia that contributes to international security and ... that’s very important to our survival,” Democratic Senator Jeff Merkley said at a Dec. 3 hearing.

Senator Todd Young, a Republican, on Wednesday will propose, as an amendment to unrelated Russian sanctions legislation, a measure that would require U.S. intelligence estimates on how Russian and Chinese nuclear forces may evolve if New START expires.

The House of Representatives foreign affairs committee also plans on Wednesday to consider similar legislation sponsored by Democratic Chairman Eliot Engel and the senior Republican, Representative Michael McCaul, a congressional aide said.

Another aide, also speaking on condition of anonymity, said the Senate bill, originally introduced by Young and Senator Chris Van Hollen, a Democrat, was being harmonized with similar language in the National Defense Authorization Act (NDAA).

The latest version of the NDAA, which passed the Senate on Tuesday, also demands intelligence estimates on Russia’s nuclear arsenal in a post-New START world but does not do so for China.

If the three bills converge, the Trump administration may find itself forced to share with Congress intelligence assessments on the implications of abandoning New START and some details on its discussions with Russia and China.

The State Department did not respond to a request for comment on the bills, whether such intelligence assessments have been done or whether the inclusion of China was a “poison pill.”

Thomas Countryman, a former acting undersecretary of state for arms control and international security, said there were limited ways to persuade China to enter into negotiations.

“You could offer to let the Chinese build up to the U.S. and Russia level (of 6,000 warheads) ... you could offer to take Russia and the U.S. down to 300. Or you could suggest to the Chinese that they stay at 300 and we’ll stay at 6,000.

“Only the second one has a chance of being accepted by the Chinese, but it’s not acceptable, unfortunately, to the Pentagon or the Kremlin,” Countryman said.