During the first 24 hours of the U.S. blockade of Iranian ports , American warships issued warnings to six vessels, prompting them to turn around, a U.S. official said Tuesday. No shots were fired in the encounters, the official said, and U.S. personnel did not board any of the vessels before they reversed course. Five of the ships were carrying oil, according to the official. The contents of the sixth vessel were not immediately clear. Two of the oil-carrying ships turned back in the first two