The estimate is up about $US10 billion from four years ago.

The weapons, known as intercontinental ballistic missiles, or ICBMs, are intended as part of a near-total replacement of the American nuclear force over the next few decades at a cost of more than $US1.2 trillion.

Some, including former Defense Secretary William J. Perry, argue US national security can be ensured without ICBMs but the Pentagon says they are vital to deterring war.

The Trump administration affirmed its commitment to fielding a new generation of ICBMs in a 2018 review of nuclear policy.

"The ICBM force is highly survivable against any but a large-scale nuclear attack," the review concluded.

"To destroy US ICBMs on the ground, an adversary would need to launch a precisely co-ordinated attack with hundreds of high-yield and accurate warheads.

"This is an insurmountable challenge for any potential adversary today, with the exception of Russia."

The current fleet of 400 deployed Minuteman missiles, each armed with a single nuclear warhead, is based in underground silos in Montana, North Dakota, Colorado, Wyoming and Nebraska.

Their numbers are governed in part by the 2010 New START treaty with Russia, which is due to expire in February.

Russia wants to extend the treaty but the Trump administration has set conditions not accepted by Moscow.

The US also is building a new fleet of ballistic missile submarines to replace the current Ohio-class strategic subs; a new long-range nuclear-capable bomber to replace the B-2 stealth aircraft; a next-generation air-launched nuclear cruise missile; and a new nuclear command and communications system.

It also is working on updated warheads, including an ICBM warhead replacement for an estimated $US14.8 billion.

The nuclear modernisation program was launched by the Obama administration and has been continued by President Donald Trump.

Joe Biden says if elected he will consider finding ways to scale back the program.

Australian Associated Press