UNITED NATIONS — A reinvigorated al-Qaeda has made “alarming” advances in Yemen, expanding its military control over several southern towns and launching a series of brazen attacks that threaten the U.S.-backed political transition there, a senior U.N. envoy warned the Security Council in a confidential briefing last week.

“The scale of these attacks serves as a stark reminder of the security threat posed by al-Qaeda,” Jamal Benomar, the U.N. special envoy for Yemen, told the 15-nation council Wednesday, according to a copy of the briefing notes obtained by The Washington Post. “Despite all counterterrorism efforts, al-Qaeda in Yemen has not retreated.”