The US breached international law when the state of Texas executed a Mexican citizen convicted of raping and killing an American girl, the UN's senior human rights official has said.
Navi Pillay cited "particular legal concerns" whether Humberto Leal Garcia, 38, had access to consular officials and a fair trial.
US Secretary of State Hillary Clinton also said she was "disappointed" that Texas carried out the lethal injection.
Leal was executed late on Thursday.
He was not told he could have access to Mexican consular officials, in violation of the Vienna Convention.
"US compliance with Vienna convention terms is absolutely critical to ensuring our own consular access and our own ability to protect Americans detained abroad," State Department spokeswoman Victoria Nuland said.
Ms Nuland said not protecting "the rights of non-Americans in the United States" could lead to a lack of US access to American citizens overseas in the future.
"This is why the secretary is concerned," she added.
Ms Pillay, the UN High Commissioner for Human Rights, who visited Mexico this week, said Texas had placed "the US in breach of international law".