U.S. Defense Secretary Robert Gates has warned NATO members they must increase their defense spending commitments or the alliance will face a “dim, if not dismal” future that might not include the United States.

Gates spoke at the Security and Defense Agenda meeting Friday in Brussels, in what is expected to be his last major policy speech before retiring at the end of the month. He said U.S. lawmakers are losing patience with covering three-quarters of defense spending for the coalition.

The U.S. defense secretary noted that all 28 NATO members voted for the mission against Libyan leader Moammar Gadhafi, but fewer than half have participated. He said some members taking part are beginning to run short of munitions after only 11 weeks, requiring the United States to make up the difference.

He bluntly warned that if NATO wants to avoid, in his words, “the very real possibility of collective military irrelevance,” member nations have to look at ways of boosting their combat capabilities.

Later Friday, Norway, which has been a partner in the air bombing of Libya, announced it will cut back its participation before ending its role in the operation on August 1. NATO on June 1 decided to extend its action in Libya until the end of September.

Gates said unwillingness or inability to contribute has also affected operations in Afghanistan, where he said the coalition has “struggled, at times desperately” to sustain a deployment of up to 45,000 troops.

He said finishing the mission in Afghanistan must be done in a united way, not dictated by each member's domestic financial situation.

Gates noted that NATO has become a two-tiered alliance, between members focusing on humanitarian and development missions and those paying the price of combat missions — a situation he described as “unacceptable.”

He said if other nations fail to step up their contributions to the more than 60-year-old transatlantic alliance, future U.S. administrations may not consider the return on its investment worth the cost.

Gates attended his final meeting of NATO defense ministers in Brussels Thursday. He said substantial military progress is being made on the ground in Afghanistan, but warned that those gains could be threatened if the transition to full Afghan government control of security is not made in a deliberate, organized and coordinated manner.