UN human rights experts on Thursday expressed "serious concern" about lethal drone attacks allegedly conducted by US forces in Yemen, that resulted in civilian casualties this month.
The UN said in a statement that 16 civilians were killed and at least 10 injured when two separate wedding processions were targeted by drones in the country on 12 December.
The victims had been mistakenly identified as members of Al-Qaeda, the UN quoted local security officials as saying.
"If armed drones are to be used, States must adhere to international humanitarian law, and should disclose the legal basis for their operational responsibility and criteria for targeting," said Christof Heyns, UN special rapporteur on extrajudicial, summary or arbitrary executions.
"Yemen cannot consent to violations of the right to life of people in its territory," he added.
The US military operates all unmanned aircraft flying over Yemen in support of Sanaa's campaign against Al-Qaeda and has killed dozens of militants in a sharply intensified campaign this year.
But Washington faces mounting criticism over the use of drones in its "war on terror".
Special Rapporteur on Torture Juan Mendez also expressed concern about the legitimacy of the December 12 airstrikes, highlighting that the states involved had a duty to investigate the reported incidents and their effect on civilians.
"A deadly attack on illegitimate targets amounts to cruel, inhuman or degrading treatment if, as in this case, it results in serious physical or mental pain and suffering for the innocent victims," Mendez said.
Heyns stressed the need for accountability when drones were used.
He called on the United States and Yemen to disclose whether they were responsible, and if so, what targeting standards were used, how many civilians were killed, and whether they plan to provide compensation for the victims' families.
Yemen is the ancestral home of Osama bin Laden and the home base of Al-Qaeda in the Arabian Peninsula (AQAP), which the United States views as the global jihadist network's most dangerous franchise.
Critics say drones in Yemen have claimed numerous civilian lives -- although the precise number remains unclear -- and have demanded an end to the secrecy surrounding their use.
Yemen's parliament this month voted for a ban on drone strikes, but experts say lawmakers have limited powers and are unlikely to impact Washington's campaign.
According to November figures from the Washington-based New America Foundation, a think-tank that has tried to track and vet the numbers, 93 strikes since 2002 have killed 684 to 891 people, including between 64 and 66 civilians.