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.
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.
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.