Their estimates on the number of U.S. airstrikes vary — from 44 to 67 since 2002, the majority of them by drones. Compiling accurate data has been even more difficult because until recently, the Yemeni military took responsibility for many strikes apparently carried out by the United States.
All three groups mark a dramatic escalation last year. The Long War Journal, for example, recorded 42 strikes in 2012, up from 10 the year before. The Associated Press has reported on nine strikes so far in 2013.
By comparison, the U.S. has carried out more than 330 airstrikes in Pakistan since 2004, though there the rate has been falling — from a peak of 117 strikes in 2010 to 46 in 2012, according to the Long War Journal's count.
Determining civilian deaths is even more difficult. The Long War Journal says it has confirmed 35 civilians and 193 militants killed by American strikes in 2012, up from six civilians and 10 militants the year before. This year, 31 militants and no civilians have been killed, according to its count. AP has reported 33 militants killed this year, with no confirmed civilian deaths.
C.I.A. director John Brennan says the strikes are only used as a last resort against suspects believed to be plotting against America.
"In short, targeted strikes against the most senior and most dangerous AQAP terrorists are not the problem, they are part of the solution," he said in remarks at the Council on Foreign Relations in August, using the initials of the group's full name, al-Qaida in the Arabian Peninsula.
The United States says al-Qaida's branch in Yemen is among the group's most dangerous and active offshoots. It has been linked to several attempted attacks on U.S. targets, including a botched Christmas Day 2009 bombing of an airliner over Detroit and explosives-laden parcels intercepted aboard cargo flights a year later.
The spike in strikes in 2012 came as the U.S. was backing a Yemeni military campaign to uproot al-Qaida militants and their radical allies who had taken over a string of southern cities and towns. The campaign, including heavy ground fighting by Yemeni troops, largely drove the militants into the mountains and countryside.
The drone strikes have taken out high-level targets in Yemen such as American-born cleric Anwar al-Awlaki, believed to have been a powerful tool for al-Qaida's recruiting in the West. Most, however, appear to target midlevel operatives. Still on the loose is AQAP's leader, Nasser al-Wahishi.
Soon after al-Awlaki's death, his 16 year-old American son, Abdulrahman, and a teenage cousin were killed in a suspected U.S. drone strike. Afterward, Yemeni protesters rallied outside the U.S. Embassy, carrying the teenagers' pictures and posters that said "U.S. tax payers kill U.S. citizens" and "Where is justice?"
On Sunday, relatives of civilians killed in strikes gathered in Sanaa to raise awareness on the deaths, in the first meeting of its kind, organized by the London-based legal action group Reprieve.
Mohammed Ahmed Bijash told the gathering that his 10-year-old daughter was killed in the southern city of Jaar when a missile hit her school during fighting last year to drive out al-Qaida militants.
"What is the crime we committed for U.S. bombs to hit our homes?" he said at the meeting. "They have turned our lives into hell."
Hussein Saeed Dahman's 16 year-old son was knocked unconscious from a drone strike in December while playing soccer with friends in in the city of Shar Hadramawt.
"The sky rained down U.S. missiles and then the kids found body parts in the soccer field," the father told AP. His son, Hamza, remains bedridden and unable to speak.
U.S. lawyer Cori Crider, who works with Reprieve, said tribal leaders she has spoken to warn the strikes only strengthen support for al-Qaida.
"There is a definitely a branch of al-Qaida here. They are real," she told AP. "The question is, What is the appropriate response to this?"
"You can't bomb you way out of an insurgency."
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