Iran is increasing its fleet of small attack boats capable of challenging warships and disrupting oil traffic in the Strait of Hormuz, the sea route for two-fifths of the world’s daily supply of crude oil, the U.S. Navy says.

Iran’s Revolutionary Guard Corps already has more than 1,000 of the speedboats “and continues to add boats armed with anti-ship cruise missiles,” said Robert Althage, spokesman for the U.S. Office of Naval Intelligence.

“Iran still states that the Iranian Revolutionary Guard Corps will employ swarming tactics in a conflict,” Mr. Althage said in an e-mail. Naval intelligence, in its latest report on threats, said an attack against U.S. forces and commercial tankers “could include over 100 boats in coordinated groups of 20 to 30 approaching simultaneously from multiple axes.”

The U.S. has two carrier groups in the Persian Gulf. The commander of these forces, Vice Admiral Kevin Cosgriff, said the attack boats have “a significant military capability.” His fear is that Iran’s central leadership might not have enough control over this Revolutionary Guard force to ensure against unauthorized attacks.

“I’m fairly comfortable that the regular Iranian navy and air force has a pretty good command-and-control system — the key word is ‘control,”‘ he said. “I don’t have the same sense with the Revolutionary Guard.”

“Is there a rigorous, disciplined chain of command where people pay attention?” Admiral Cosgriff said. “In some instances, the answer would be yes. In other instances I’ve had some concern that people may be prone to miscalculation.”