WASHINGTON -- Advancing what could become a near-total rebuild of the U.S. nuclear weapons arsenal, the Air Force on Friday solicited industry proposals to build a new fleet of land-based nuclear missiles as well as replacements for its air-launched nuclear cruise missile force.

The two projects are part of a broader modernization of the nuclear arsenal expected to cost hundreds of billions of dollars over 30 years. The plans have broad support in Congress, although some, including Sen. John McCain, R-Ariz., have questioned the need to replace all three "legs" of the nuclear triad -- the submarines, long-range bombers and land-based missiles that were developed by the Pentagon during a Cold War arms race with the Soviet Union.

The Air Force operates two of the three legs the nuclear arsenal -- the bombers and the Minuteman 3 intercontinental ballistic missiles, or ICBMs, that are ready for launching on short notice from underground silos in five states.

On Friday the Air Force asked that industry contractors submit proposals for a new-generation ICBM, and said it plans to award the first contracts next summer. It would replace the existing fleet of about 450 deployed Minuteman 3 ICBMs, starting in 2027. The estimated cost is $62.3 billion, according to Leah Bryant, spokeswoman for the Air Force Nuclear Weapons Center.

An Air Force press release quoted Gen. Robin Rand, who heads Air Force Global Strike Command, as saying the Minuteman 3, which was first deployed in 1970, will have "a difficult time surviving" air defences foreseen for 2030 and beyond.

The Air Force also requested contractor proposals for a new-generation nuclear cruise missile to replace the existing AGM-86B cruise missile, which was fielded in the early 1980s. It provided no cost estimate for the replacement missile.

Critics of buying a new nuclear cruise missile include a former secretary of defence, William Perry, who has called on President Barack Obama to scrap the project.

The Navy wants to build new nuclear-missile submarines to replace its aging fleet of Ohio-class subs, and the Air Force is planning a new fleet of nuclear-capable long-range bombers to replace the B-52.

Daryl Kimball, executive director of the Arms Control Association, said the broad rebuild of the nuclear arsenal is financially unsustainable.

"The Air Force could save billions by refurbishing and extending the life of the existing Minuteman 3 well beyond 2030 rather than building a completely new and more deadly missile," he said, adding: "The Air Force does not need a costly new and more capable nuclear-armed cruise missile, especially if the new long-range penetrating bomber is truly penetrating. We are seeing a return to the days of nuclear excess and overkill."