WASHINGTON - More than two and a half years after US commandos shot dead Al-Qaeda figurehead Osama bin Laden, the global extremist network is more dangerous than ever, American experts and counterterrorism officials warned this week.
Thanks notably to a flood of recruits flowing to join Al-Qaeda-linked jihadist forces fighting in Syria's civil war, the group is back on its feet, and securing territory from which it could once more threaten Europe and the United States.
Bin Laden's former lieutenants in Al-Qaeda's historic leadership have been killed by US Special Forces or in drone strikes, or else are isolated and on the run in the tribal badlands on the Afghan-Pakistan border.
But armed groups in Syria, Somalia, Yemen, Libya and West Africa have flocked to his banner and Al-Qaeda is rebuilding its influence and recruiting fighters across the region.
"Their leadership has been hit very hard, but this brand is still growing. And it's growing from an increased number of safe havens," said retired US Marine Corps general James Mattis.
Between 2010 and earlier this year, Mattis led US Central Command, in charge of prosecuting Washington's long war against extremists in the Middle East, Southwest Asia and the Horn of Africa.
Now he has hung up his uniform, but admits the war is far from over, warning: "The congratulations that we heard two years ago on the demise of Al-Qaeda were premature and are now discredited."
Speaking at the Jamestown Foundation's annual conference on terrorism in Washington, Mattis said: "Al-Qaeda is resilient, they adapted. We have to think strategically before we act, not only act tactically."
Bin Laden's death in May 2011 triggered a wave of optimism that the United States and its allies might have broken the back of the jihadist threat, but today officials here are under no illusions.
Since the audacious commando strike that took out Al-Qaeda's apparently largely symbolic chieftain, the black banner of his movement has been raised more widely than ever.
Militants inspired by or linked to Bin Laden's brand of armed jihad have sacked a US consulate in Libya and stormed a shopping mall in Kenya.
Attacks are on the rise once again in Iraq, Al-Qaeda has reportedly begun operating in Egypt's Sinai desert and violent extremist groups are now the most powerful elements in the rebel coalition fighting in the Syrian civil war.
Bruce Hoffman, director of the Center for Peace and Security Studies at Georgetown University, told the Jamestown conference that collapsing states in the Middle East were opening up space for extremists.
"The oxygen that Al-Qaeda depends on is access to sanctuaries and safe haven. And unfortunately over the past two years it gained greater access to more ungoverned spaces," he said.
"The success of the attack in Nairobi and earlier in Mumbai suggests that this groups have now within their capacity the ability to fulfil one of Bin Laden's last commands or operational desires, which was to stage Mumbai-style attacks in Europe."
For the experts gathered in Washington, Syria's civil war - which has attracted Islamist volunteers from Muslim communities in Europe as well as Arab countries - has worked greatly to Al-Qaeda's advantage.
"The Al-Qaeda affiliated groups have created an alliance which disposes of 45,000 guerrilla fighters across the country," said David Kilcullen, a renowned counterinsurgency expert who has advised US forces in the field.
"It's a very significant number, almost twice as many as we see in terms of Taliban fighters in Afghanistan," he said. "We're seeing a recovery on all fronts for Al-Qaeda."
For Bruce Riedel, a three-decade CIA veteran who now works for the Brookings Institution, Al-Qaeda's resurgence is proof that it has managed to ride out and ultimately profit from the revolts that have roiled the Middle East and North Africa.
"Al-Qaeda's narrative was challenged in 2011 by the Arab Spring. Peaceful demonstrations succeeded in toppling dictators. Al-Qaeda's narrative was at risk. Terror had not produced change, Twitter had," Riedel said.
"But today, everything is different. Al-Qaeda's narrative is validated in 2013, most notably in Egypt. The counterrevolution has succeeded, the army has overthrown the elected government.
"For those who want to join Al-Qaeda's movement, events in Cairo, in Damascus have validated what they long said: Jihad is the only solution to the problem of change in the Muslim world today."