WASHINGTON—U.S. defense leaders are increasingly concerned that Israel is preparing to take military action against Iran, over U.S. objections, and have stepped up contingency planning to safeguard U.S. facilities in the region in case of a conflict.

President Barack Obama, Defense Secretary Leon Panetta and other top officials have delivered a string of private messages to Israeli leaders warning about the dire consequences of a strike. The U.S. wants Israel to give more time for the effects of sanctions and other measures...