Islamist militias in Libya took control of nearly a dozen commercial jetliners last month, and western intelligence agencies recently issued a warning that the jets could be used in terrorist attacks across North Africa.

Intelligence reports of the stolen jetliners were distributed within the U.S. government over the past two weeks and included a warning that one or more of the aircraft could be used in an attack later this month on the date marking the anniversary of the Sept. 11, 2001, terrorist attacks against New York and Washington, said U.S. officials familiar with the reports.

“There are a number of commercial airliners in Libya that are missing,” said one official. “We found out on September 11 what can happen with hijacked planes.”

The official said the aircraft are a serious counterterrorism concern because reports of terrorist control over the Libyan airliners come three weeks before the 13th anniversary of 9/11 attacks and the second anniversary of the Libyan terrorist attack on the U.S. diplomatic compound in Benghazi....