American drones strikes may have killed as many as 40 Yemeni civilians over the past year, the UN reported on Monday, offering a tally of the human cost of the long-running US campaign against al Qaeda in Yemen, which has continued amid the chaos of country's current war.

The data on drone strikes came from the latest report on Yemen issued by the UN's Office of the High Commissioner For Human Rights (OHCHR), which compiled accounts of human rights violations from July 1, 2014 to June 30 of this year.

The US first launched armed unmanned aerial vehicles (UAVs) over Yemen in 2002, but the bulk of strikes carried out by the aircraft have taken place since since 2011. According to figures maintained by the Bureau of Investigative Journalism's Drone War program, as many as 101 civilians have been killed by confirmed drone strikes in Yemen, plus 26 to 61 others killed by "possible extra drone strikes." Between 156 and 365 civilians have also been killed in other covert missions since 2002, according to the group.

If accurate, the UN's estimates would represent a significant rise in confirmed civilian casualties in the country as a result of drone strikes.

"OHCHR received reliable information indicating that as many as 40 civilians, including a child, may have been killed during the period under review as a result of drone attacks in Al-Baida, Al-Jawf, Marib and Shabwah," the OHCHR report states.

Chris Woods, an investigative journalist with the airstrike-tracking website Airwars, said it is well known that the US is the only country operating armed drones in Yemen, particularly after Houthi rebels forced the Yemeni government to flee to Saudi Arabia's capital Riyadh in March. The US claims to exclusively target alleged members of al Qaeda in the Arabian Peninsula (AQAP), widely considered the terror group's most dangerous franchise. In recent years, AQAP has had a hand in several of the most notorious planned terror attacks on aircraft, including a failed bombing attempt on Christmas Day in 2009 when a passenger tried to set off plastic explosives sewn to his underwear.

Though the US operates deadly drone-based counterterrorism operations in other countries, including Pakistan and Somalia, Woods said the strikes in Yemen appear to be carried out with fewer safeguards.

"The drone ops we tend to see in places like Yemen, I am absolutely sure they are using a different rule book because they are counter-terrorism targets and they're deemed to be higher value targets," Woods told VICE News. "That places civilians at greater risk on the ground. We see more civilians killed when it's counter-terrorism ops, because the targets are deemed as a threat to the US homeland and therefore there's a greater tolerance of collateral damage."

As Yemen's war worsened this year, US military forces in the country, who were involved in targeting drone strikes, were forced to leave. That development, said Woods, could make the strikes even less accurate, and lead to greater civilian casualties.

"The UN numbers are deeply worrying," said Woods. "Time and again we find that civilian casualties on the ground are completely counterproductive to America's long term strategic interests in the region."

The drone data was only a small part of OHCHR's reporting, which largely pertained to the ongoing conflict.

The UN cited both Houthi rebels and allies loyal to former Yemeni President Ali Abdullah Saleh as responsible for 508 civilian casualties, predominantly due to indiscriminate shelling. The Saudi-led coalition that has fought the Houthis and their allies since they drove Yemeni President Abd Rabbuh Mansur Hadi to Riyadh in March were cited for 941 civilian deaths. Al Qaeda, which has capitalized on the ensuing chaos to capture some territory, claimed responsibility for attacks that killed 24 civilians — fewer non-combatants than the UN says may have been killed by the American drone program.

Last Thursday, the UN announced that Hadi, along with representatives from the Houthis and Saleh's General People's Congress, had agreed to commence peace talks this week. Over the weekend, however, Hadi said he had no such intentions, and refused to broach the question of peace talks until the Houthis retreat from all territory they have seized and lay down arms seized during their insurrection.

The abrupt turnaround recalled a July announcement about a humanitarian pause that the UN claimed was agreed to by all parties. That pause was violated almost immediately and wholly ignored by all sides.

Peace talks, unlike the pause, do not require a cessation of hostilities, and it remained unclear what made Hadi reportedly change his mind.

An email, written by UN special envoy for Yemen Ismail Ould Cheikh Ahmed, and leaked earlier this month, showed that officials felt the Saudis were not invested in peace talks, and appeared intent of defeating the Houthis militarily. On Monday, some UN officials said that they believed Hadi was pressured by the Saudis.

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