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.”

Members of Al-Qaeda in the Arabian Peninsula, as the group’s Yemen branch is known, have been “intensifying their attacks” against government targets since the election last month of Yemen’s new president, Abed Rabbo Mansour Hadi, Benomar told the council. Hadi, the former vice president, replaced Ali Abdullah Saleh after 33 years in power in a transition brokered by Arab leaders and supported by the United States.

On inauguration day, Feb. 25, al-Qaeda struck a presidential palace in the provincial capital of Mukalla, killing 26 officers. More recently, Benomar said, al-Qaeda has launched a series of attacks on military bases in the south, killing more than 180 soldiers and capturing heavy weapons. Dozens of soldiers, he added, were reportedly paraded through the town square of Jaar, which has been held by al-Qaeda for several months.

The gloomy account underscored the challenges of confronting an enemy that has shown resilience since the United States succeeded last year in killing its leader, Osama bin Laden, and Anwar al-Awlaki, a U.S.-born cleric who was a skilled propagandist and who had a key role in external operations for the Yemen affiliate, according to U.S. officials. The report also highlighted al-Qaeda’s effort to plant roots in Yemen, forming alliances with local tribal leaders while providing basic services to communities.

“This recent campaign launched by al-Qaeda poses a new challenge to the new government,” Benomar said. “It continues to control strategic territory in the south and evolve as a major threat. The group is also increasing their social leverage by ensuring security, administering justice and providing basic social services in the local communities under their control.”

Benomar said that Yemen’s new president has made a “clear commitment” to combat al-Qaeda but that “many Yemenis doubt how progress on this front could be achieved with a fractured army with multiple loyalties.”

On Saturday, the military launched airstrikes that killed 18 militants linked to al-Qaeda in central Yemen and wounded nine in the south, according to the Associated Press.

In response to Benomar’s briefing, a U.S. official, speaking on the condition of anonymity to discuss intelligence, conceded that the threat posed by al-Qaeda in Yemen “is real,” allowing the terrorist organization “to seize territory in southern Yemen.”

But “we believe the unfolding transition process in Yemen will undercut popular support for” al-Qaeda, the official said.

U.S. intelligence officials say that al-Qaeda’s power has been diminished in the aftermath of bin Laden’s death. James R. Clapper, the U.S. director of national intelligence, told the Senate Select Committee on Intelligence in a prepared statement in January that al-Qaeda’s leadership structure will grow increasingly decentralized over the coming two to three years, with the Pakistani-based leadership “diminishing in operational importance.”

Clapper’s statement noted that regional affiliates, including the one based in Yemen, will continue to pose a serious threat to U.S. interests, even with Awlaki dead.

“AQAP remains the most likely to attempt transnational attacks,” Clapper said. “His death probably reduces, at least temporarily, AQAP’s ability to plan transnational attacks, but many of those responsible for implementing plots, including bombmakers, financiers, and facilitators, remain and could advance plots.”

Benomar said that the political “transition remains on track” in Yemen and that “each milestone set out in the agreement has been achieved within the allotted time frame.”

Yet he added: “To judge progress of the transition merely by the fulfillment of the markers in the agreement, however, would be simplistic and misleading. Rather, long-term viability of the transition must be viewed against the many and profound challenges Yemen continues to face on the security, political and economic fronts.”

Yemen remains among the poorest countries in the world, registering the second highest rate of chronic malnutrition in the world after Afghanistan. Half a million children, according to Benomar, are likely to die or endure life-long symptoms from chronic malnutrition, and more than 3 million are in need of humanitarian assistance.

The United States has been engaged in a low-intensity conflict against al-Qaeda in Yemen ever since militants mounted their October 2000 attack against the USS Cole in the port of Aden.

It also has served as an early testing ground for the U.S. government’s use of drone aircraft to kill al-Qaeda members. Last year, the CIA set up a drone base in the region, which was used in the killing of Awlaki.

Al-Qaeda’s ranks in Yemen have grown in recent months, with fresh recruits arriving from Afghanistan, Somalia, Pakistan and other countries in North Africa and the Middle East, according to Richard Barrett, the head of the U.N. panel that monitors the group’s global activities. He said it also has “corralled” other Yemeni groups under an organization, Ansar al-Sharia, which means supporters of sharia, or Islamic law.

Barrett said the movement has benefited from the political turmoil over the past year, which concentrated attention on Saleh’s gradual weakening and departure.

While the movement has demonstrated an impressive capacity to “mount these quite spectacular attacks,” Barrett said it is unlikely al-Qaeda’s local allies will embrace global jihad. “There is not the ideological cement to bind them come what may,” he said.