Since the U.S. blockade of Iranian ports began , American warships issued warnings to 10 vessels, prompting them to turn around, U.S. Central Command said Wednesday.

No shots were fired in the encounters, according to a U.S. official, and U.S. personnel did not board any of the vessels before they reversed course.

At least five of the ships were carrying oil, according to the official. Two of the oil-carrying ships turned back in the first two hours after the blockade went into effect Monday morning. An Iranian-flagged cargo vessel tried to evade the U.S. blockade but was redirected by the Navy destroyer USS Spruance.

President Donald Trump announced the blockade Sunday after negotiations with Iran failed to produce an agreement to end the war. Since the United States and Israel launched attacks on Iran in late February, traffic through the Strait of Hormuz has been at a near-standstill, sparking a rise in the prices of oil and other key goods.

The U.S.…