WASHINGTON (Reuters) - The U.S. Air Force asked industry on Friday for proposals to replace the Minuteman III intercontinental ballistic missile and the nuclear cruise missile as the military moves ahead with a costly modernization of its aging atomic weapons systems. The Air Force said in a statement it expected to award up to two contracts for a new ICBM weapons system, or ground-based strategic deterrent, sometime next summer or fall. It also expected to award up to two contracts in the same time frame for a new nuclear cruise missile, or long-range standoff weapon. Modernization of the U.S. nuclear force is expected to cost more than $350 billion over the next decade as the United States works to replace its aging systems, including bombs, nuclear bombers, missiles and submarines. Some analysts estimate the cost of modernization at $1 trillion over 30 years. The new ICBM system would be a follow-on to the Minuteman missile, whose launch systems and physical infrastructure first became operational in the mid-1960s. The system has been upgraded over the years, but much of the infrastructure is original, the Air Force said. The most recent versions of the Minuteman III date from the late 1990s and early 2000s and had an intended 20-year life span, the Air Force said. The missile will “face increased operational and sustainment challenges until it can be replaced,” it said. “This request for proposals is the next step to ensuring the nation’s ICBM leg of the nuclear triad remains safe, secure and effective,” said Major General Scott Jansson, who leads the Air Force program office for strategic systems. Opponents of replacing the nuclear cruise missile have argued that its missions could be handled by other legs of the triad. Others say it is an unnecessary expense at a time of shrinking budgets and smaller deployed nuclear arsenals. The military insists the new cruise missile is needed to enable older bombers to deliver nuclear weapons to targets whose air space is heavily defended and difficult to reach with gravity bombs. The missile is “needed to replace the aging air launched cruise missile, which has far exceeded its originally planned service life ... and is required to support our B-52 bomber fleet,” Admiral Cecil Haney, the head of U.S. Strategic Command, told lawmakers earlier this year. 