By Jonathan Landay

WASHINGTON (Reuters) - While President Donald Trump has suggested that the United States must expand its nuclear arsenal, many experts say U.S. nuclear forces are unrivaled and will remain so as they undergo a modernization program that could cost more than $1 trillion.

In an interview with Reuters on Thursday, Trump said the United States has "fallen behind on nuclear weapon capacity." He pledged to ensure that, "We're going to be at the top of the pack."

While Moscow currently deploys 200 more strategic nuclear warheads than the United States does, both countries are bound by the 2010 New START treaty to slash their deployed strategic warheads to no more than 1,550 each by February 2018, the lowest level in decades. The accord also limits their deployed land- and submarine-based missiles and nuclear-capable bombers.

However, nuclear weapons experts say, the 30-year modernization program, which maintains many existing weapons and their computers, communications, electronics, and other systems, is more important than having as many warheads as Russia has.

Trump "says we can't fall behind. Fall behind who and how?" said Stephen Schwartz, an independent nuclear weapons expert. "It is not clear to me, and it's not clear to many of my colleagues" what the president is talking about when he pledges to expand U.S. nuclear weapons capacity, Schwartz said.

Moreover, the U.S. advantage is based not on the numbers of warheads it can field compared to Russia, but on more advanced delivery systems.

Most of Moscow's nuclear force - now being modernized, as well - is comprised of land-based ICBMs whose locations it discloses to Washington under arms control accords, said Schwartz, former publisher of the Bulletin of the Atomic Scientists leading journal.

The United States maintains an "invulnerable" fleet of nuclear-armed submarines beneath the Atlantic and Pacific Oceans that are immune from detection, he said.

In contrast, Russia's accident-prone missile submarines rarely conduct their "deterrence patrols" far from their docks.

RUSSIA WAY BEHIND

In a fiscal 2012 report to Congress, the Pentagon assessed that even if Russia broke out of the limits imposed by the New START treaty and deployed more nuclear weapons, it could not gain strategic advantage over the United States.

“The Russian Federation, therefore, would not be able to achieve a militarily significant advantage by any plausible expansion of its strategic nuclear forces, even in a cheating or breakout scenario under the New START Treaty,” the report said.

Trump “clearly needs to get a briefing on the capacity of U.S. nuclear forces,” said Hans Kristensen, director of the Nuclear Information Project at the Federation of American Scientists, a leading arms control organization.

The 30-year U.S. modernization program is intended to maintain the American advantage. It upgrades and extends the lives of every U.S. nuclear weapons system – intercontinental ballistic missiles, long-range bombers and submarines – and their warheads.

In addition, the Pentagon has drawn up plans to replace many systems with a new bomber, new ballistic missile submarine, and new missiles.

Many lawmakers and experts are concerned about the price tag of the effort, given the high cost of repairing and modernizing the country's conventional military forces and the budgetary demands of non-military programs.

The costs of the modernization program has been estimated at as much as $1 trillion over the next 30 years.

But a report published this month by the Congressional Budget Office indicated that the overall cost is rising.

Nuclear force plans set out by the Pentagon and the Energy Department – the caretaker of the U.S. nuclear arsenal – in their fiscal 2017 budget requests would cost an estimated $400 billion during only the 10-year period ending in fiscal 2026.

(Reporting by Jonathan Landay; Editing by John Walcott and Alistair Bell)