U.S. Defense Secretary Robert Gates has warned that NATO members that 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 think tank 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.

Gates 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 who are taking part are beginning to run short of munitions after only 11 weeks, requiring the United States to make up the difference.

He 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.

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

Gates attended his final meeting of NATO defense ministers in Brussels Thursday and 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