BEIRUT—A U.S. conclusion that Iran was behind recent strikes on oil tankers has reignited concern about Iran’s ability to wage guerrilla warfare in one of the world’s most vital waterways.

The U.S. believes Iran used mines to strike four tankers in the Gulf of Oman this month, White House national security adviser John Bolton said Wednesday, without providing more details. Iran called Mr. Bolton’s allegations “laughable.” Both sides say they don’t want war.

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