WASHINGTON, Nov 5 (Reuters) - Americans are concerned about Iran's nuclear program but split on whether the United States should take military action to shut it down if diplomatic efforts fail, according to a USA Today/Gallup Poll released on Monday.

Seventy-three percent of those surveyed said the United States should use economic sanctions and diplomacy to stop Iran's nuclear program and 18 percent favored military action, the poll found.

However, 46 percent said military action should be taken either now or if diplomacy fails while 45 percent ruled out a military strike altogether, USA Today said.

Republicans were twice as likely as Democrats to endorse taking military steps, the poll said.

President George W. Bush has suggested a nuclear-armed Iran could lead to World War Three. Bush last week imposed sanctions on Iran's military and its financial sector, hoping to increase pressure on Tehran to stop uranium enrichment and curb what the U.S. government says are terrorist activities.

Iran has so far refused to heed United Nations demands to halt nuclear work that has both civilian and military uses.

Three of four Americans polled said they were concerned the United States "will not do enough to prevent Iran from developing nuclear weapons," USA Today said.

An equal number also said they were concerned that the United States will be too quick to use military force against Iran.

The telephone survey of 1,024 adults was taken Friday through Sunday. The margin of error is plus or minus 3 percentage points.

(Reporting by JoAnne Allen, editing by Doina Chiacu)

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