Al-Qaida's been taking a beating in recent weeks as the U.S.-backed regime in Sanaa has driven militants out of captured cities in the south but they're striking back with a crippling wave of assassinations against Yemeni intelligence services.

A surge in missile attacks by U.S. unmanned aerial vehicles against the leadership of al-Qaida in the Arabian Peninsula, arguably the most dangerous of al-Qaida's branches, has also taken a toll of key jihadist figures.

At least 36 U.S. drone strikes have been carried out in Yemen this year, more than the combined total for the previous four years.

The airstrikes have taken out important leaders and it would seem the jihadists' assassination campaign is payback for these attacks. Initially the killings involved bombings concentrated in the southern provinces of Abyan and Shabwa, including the port city of Aden, but they switched to Sanaa and were carried out by motorcycle-riding gunmen.

The close-in, high-speed Sanaa hits were so similar, security officials suspect they were carried out by a special al-Qaida death squad.

"The Sanaa attacks share several unique hallmarks," observed the U.S global security consultancy Stratfor.

"The consistency of the tactics employed and the similarities among the officials targeted strong suggest that a coordinated group of militants have carried out the attacks ...

"The group has demonstrated an unusual level of tactical efficiency ... Perhaps most telling, the officials killed were trained intelligence officers who were likely already on alert due to previous assassinations."

AQAP announced its payback operation June 15, vowing that if the Sanaa regime "would not stop mounting campaigns in Abyan to annihilate its people, we will certainly transfer the battle to other regions and to major cities like Sanaa, Aden and others."

Three days later, a suicide bomber blew set off a device outside the Aden home of Brig. Gen. Salem Ali al-Qutn, military commander of Yemen's southern region who had commanded the June offensive against al-Qaida.

In the weeks that followed, the jihadists struck again and again. None of the killers have been caught.

Their targets were leaders of the government's counter-terrorism campaign, intelligence officers and senior military officers, primarily those involved in the military's southern operations that drove AQAP out of cities they seized earlier in the year.

The killings also were apparently intended to sabotage efforts by Yemeni President Abd Rabboh Mansour Hadi, a former general who took over in February after the United States and Saudi Arabia ditched longtime dictator Ali Abdullah Saleh, to consolidate power and restructure Yemen's fractured armed forces.

On July 3, Col. Mohammed al-Qutani of the Political Security Intelligence unit was killed by a bomb planted in his car in Sanaa.

Three weeks later, Col. Abdallah al-Maouzaei, a senior police commander tasked with hunting militants, was killed in a similar manner in Aden. He'd survived three attempts to kill him.

On Aug. 10, Brig. Gen. Omar Barashid, head of the Command and General Staff College, was slain in a roadside bombing in southeastern Hadramaut province.

After that, the assassins focused on targets in Sanaa. These have been mostly gun attacks, almost identical in style, that targeted intelligence and security officers.

On Aug. 30, gunmen on a motorcycle in Sanaa's Habra quarter shot Col. Yahya Badi, a senior intelligence officer with the Political Security Organization.

Brig. Gen. Abdullah al-Ashwai, one of the most prominent officers in the PSO who had thwarted several AQAP attacks, was killed by motorcycle gunmen in the city's al-Safia district Sept. 24.

On Oct. 11, motorcycle shooters killed Qassem Aqlani, long-serving link man between the U.S. counter-terrorism units and the PSO who was based in the U.S. Embassy.

Five days later, a speeding gunman on a motorcycle killed Gen. Khaled al-Hashim, a former Iraqi army officer, one of many hired by Yemeni intelligence after the fall of Saddam Hussein in 2003.

"In their efforts to restore the Islamist militant movement, AQAP still sees possibilities in the ongoing political turmoil," observed the Jamestown Foundation, a Washington think tank that monitors global terrorism.

"The complex Yemeni transition, the difficulties the Hadi administration is facing in reorganizing the security establishment and the lack of government control in large swathes of Yemeni territory all represent major opportunities for AQAP to expand its fighting capabilities and try to reclaim its lost territory."