Iran has begun laying mines in the Strait of Hormuz, the Persian Gulf channel that carries 20 percent of the world’s oil, according to U.S. officials, an effort that could further complicate American efforts to restart shipping there. While the U.S. military said it had destroyed larger Iranian naval vessels that could be used to quickly lay mines in the strait, Iran began using smaller boats for the operation on Thursday, according to a U.S. official briefed on the intelligence.