By Tim Lister and Paul Cruickshank
The revelation that al Qaeda leader Ayman al-Zawahiri has been communicating directly with the group's Yemeni franchise about future operations is causing plenty of consternation among western counter-terrorism officials.
It suggests a heightened level of co-ordination between al Qaeda 'central' and its branches, and an initiative by Zawahiri to leverage instability in places far away from his hideout – thought to be somewhere along the Pakistan-Afghan border.
There are still few confirmed specifics about the nature of the plot that Zawahiri was supposedly discussing with Nasir al-Wuhayshi, the leader of al Qaeda in the Arabian Peninsula (AQAP). But he has clearly identified that group as an effective affiliate, possibly the one best placed to attack U.S. interests directly.
U.S. officials were concerned that a plot was timed to go into operation to coincide with the end of Ramadan, which has often been a period of increased terrorist activity. The Muslim holy month ends on Wednesday.
"The security threat level in Yemen is extremely high," the State Department announced in a travel advisory on Tuesday, while repeating guidance that all U.S. citizens should leave the country. Britain suspended operations at its embassy in Sanaa indefinitely and has withdrawn its staff.
The Yemeni government also acknowledged the threat, issuing a list of the 25 most dangerous terrorists in the country and promising a reward for information leading to the capture of any of them. It has also bolstered security throughout the capital.
Over the past 13 years, al Qaeda has seen Yemen as one of its most promising hunting grounds – beginning with the attack on the USS Cole in the port of Aden in 2000.
In the middle of Ramadan in September 2008, Wuhayshi orchestrated a complex attack on the U.S. embassy in Sanaa – one of the most heavily fortified diplomatic missions in the world.
Although the attackers did not breach the perimeter, at least six Yemeni guards and four civilians were killed in an assault that involved suicide bombers driving vehicles packed with explosives, snipers and rocket propelled grenades.
More recently, AQAP was responsible for the attempted "underwear bombing" that targeted a U.S. airliner over Detroit and the successful dispatch of printer bombs as air cargo destined for the United States.
Only a Saudi intelligence tip at the last moment led to the discovery of those explosives. Another plot was thwarted last year because the intended suicide bomber was a Saudi informant.
Last September 11, three or four Yemeni AQAP operatives participated in the attack on the U.S. diplomatic mission in Benghazi, several counter-terrorism sources have told CNN.
Western intelligence officials suspect they were dispatched by the group to take part, but have not ruled out that they were already in eastern Libya and decided to join in.
The Yemeni security forces, extensively reorganized by President Abed Rabbo Mansour Hadi over the past 18 months, have been able to recapture territory in southern Yemen that was briefly held by al Qaeda, forcing AQAP fighters - who are known in Yemen as Ansar al Sharia – to retreat into more mountainous provinces such as Hadramaut in the southeast.
Extensive U.S. drone attacks over the past two years – some launched from a new desert base in southern Saudi Arabia – have also whittled away al Qaeda's infrastructure in Yemen.
The New America Foundation estimated last month that some 30 senior operatives have been killed, and there has been a spate of further attacks since.
Agence France Presse and other agencies reported Tuesday the latest drone attack in eastern Marib province killed four militants, including one of those on the latest list issued by the Yemeni government, Saleh al-Tays al-Waeli.
AQAP confirmed last month that its deputy leader, Said al-Shihri, had been killed. Last year, a drone killed Fahd Mohammed Ahmed Quso, an al Qaeda operative believed to have plotted the bombing of the U.S. destroyer Cole in 2000 that killed 17 American sailors. And the militant U.S. born cleric, Anwar al Awlaki, was among several killed in a drone attack in Jawf province in September 2011.
But the drone campaign has also provoked considerable resentment among Yemen's tribes – especially as several have claimed civilian lives. In some provinces of Yemen, al Qaeda fighters are tolerated as no worse an evil than the Yemeni security forces and because they pose no local threat.
The group's takeover of swaths of territory in southern Yemen in 2011 was initially welcomed by some of local population.
In areas where it was strong, Ansar al Sharia delivered basic services and capitalized on a widespread distrust of the government in the marginalized south.
But its imposition of Taliban-style Islamic justice eventually made it unpopular with many. According to Amnesty International, its punishments included summary killings, amputations, and even public crucifixion.
Hadi's offensive against the group, extensively supported by U.S. and Saudi intelligence, has taken its toll. Last summer, the group lost control of the last sizable towns it controlled in the south.
AQAP has not mounted a large-scale suicide attack on the security forces since May 2012, when more than 100 soldiers were killed by a suicide bomber as they trained for a parade in Sanaa. Also, it appears that the AQAP leadership fears extensive infiltration by Saudi informants.
Even so, AQAP has continued to target military officers for assassination throughout the country.
The group's recent attacks have included a suicide bombing on a pro-government militia in the south in March that killed 12, and an attempted suicide bombing attack on a gas pumping facility in the port city of Balhaf in June. In July, several soldiers were killed by a bomb in Sana'a after a lull in attacks in the capital.
Many of AQAP's operatives, including its leadership, retreated into remote areas after the Yemeni military offensive last year and regrouped.
AQAP has another historic advantage: well established networks of sympathizers and safe-houses in Sanaa, and its major port, Aden.
Some mosques in the capital are known as recruiting and contact centers. An ambitious attack against Western interests in Sanaa would still seem to be within AQAP's capabilities. To prepare for such an attack, the group conducted video reconnaissance of buildings linked to U.S. personnel in the city in late 2011, according to a source who has seen the video.
AQAP remains a resilient organization, bolstered by the addition of hardened Saudi jihadists, some of whom had spent time at Guantanamo Bay. In Ibrahim al-Asiri, a young Saudi with a background in chemistry, it has al Qaeda's most expert bomb-maker.
Just how Zawahiri is communicating with Wuhayshi is fascinating. It seems unlikely they would be in touch directly: the security risks are too great. A eulogy for al-Shihri noted he had become careless in making telephone calls.
Counter-terrorism experts believe Zawahiri and Wuhayshi may be using flash drives. So Zawahiri would compose a message and save it to a drive. A courier – or series of couriers – would then take it to a place with an Internet connection and send it to a trusted source in Yemen. And Wuhayshi would use the same technique. It is time-consuming and cumbersome, and dependent on trusted couriers.
Such a method was used frequently by Osama bin Laden from his hide-out in Abbattobad in Pakistan, as he tried to influence and direct operations around the world. It was also employed by al Awlaki, who spent the last two years of his life moving among Yemen's more remote provinces.
For Hadi, who is trying to establish a national dialogue in Yemen and desperately seeking economic assistance after several years of steep economic decline, the current headlines are an unwelcome distraction.
But the United States regards Hadi as a solid ally after all the prevarications and nepotism of his predecessor, Ali Abdullah Saleh, and is likely to redouble its efforts in the face of a burgeoning new partnership within al Qaeda.