The United States will keep its current force of 450 land-based nuclear missiles but remove 50 from their launch silos as part of a plan to bring the U.S. into compliance with a 2011 U.S.-Russia arms control treaty, the Pentagon said Tuesday.

The resulting launch-ready total of 400 Minuteman 3 intercontinental ballistic missiles would be the lowest deployed ICBM total since the early 1960s.

The decisions come after a strong push by members of Congress from the states that host missile bases — North Dakota, Wyoming and Montana — to not eliminate any of the silos from which the missiles would be launched.

The decision to put 50 of the missiles in storage but not eliminate any of their launch silos meant the Pentagon had to make steeper reductions in the Navy's sea-based nuclear force in order to comply with the New START, or Strategic Arms Reduction Treaty, by 2018. The Navy will reduce the number of deployed and non-deployed submarine-launched ballistic nuclear missiles to 280 from the current 336.

The Navy has 14 Ohio-class submarines armed with missiles but only 12 will count as deployed because two will be undergoing long-term maintenance at a given time during the 10-year life of the New START treaty. The Navy is embarking on a long-term program to build a replacement for the current fleet.

The other "leg" of the U.S. nuclear force, the Air Force strategic bombers, will be trimmed from the current deployed total of 93 to 60, with an additional six available in a non-deployed status. The 60 will be comprised of 19 B-2 stealth bombers and 41 B-52H Stratofortress heavy bombers.

Thus the administration will remain within the New START limit of 700 deployed strategic nuclear weapons with 400 ICBMs, 240 sub-launched missiles and 60 bombers. Russia already is well below the 700-deployed weapon limit; at the most recent reporting period, last October, Russia had 473.

The 400 deployed ICBMs would be the lowest total since 1962, according to a history of the force by Hans Kristensen, a nuclear weapons expert at the Federation of American Scientists. He says the U.S. had 203 deployed ICBMs in 1962, with the force expanding rapidly to 597 the following year and topping 1,000 in 1966. It has been between 550 and 450 since 1991.

The Obama administration spent months figuring out how to apportion the reductions required to comply with the New START treaty. In the meantime, the ICBM force came under heavy scrutiny for a variety of problems, including low morale, leadership failures and investigations over exam-cheating and drug use among launch officers.

Some question the value of retaining ICBMs, although President Barack Obama has committed to keeping them as part of the nuclear "triad" of forces that can be launched from land, sea and air.

The Pentagon said Tuesday it probably will cost about $300 million to implement all the announced changes required to comply with New START by 2018. About two-thirds of the cost will be for altering some of the missile tubes aboard Navy submarines so they can no longer launch ballistic missiles.

The nuclear sub fleet is far more costly to operate than either the land-based missiles or the bombers, but its strategic advantage is the relative invulnerability of the submarines while at sea, and thus their ability to survive a first strike.

The New START treaty also requires both Russia and the U.S. to reduce to 1,550 the number of nuclear warheads associated with the deployed missiles and bombers. The Pentagon has not spelled out how it will do that, but analysts have said they believe the breakdown will be: 1,090 warheads aboard subs, 400 on land-based missiles and the 60 bombers counting as one warhead each.

Obama announced last summer that the U.S. would be ready to reduce its total warheads by another one-third, to about 1,100, in a new round of negotiations with Russia. But there is scant chance of that happening anytime soon, especially with the crisis over Russian intervention in Ukraine.