The European Union has said it "deeply regrets" the first federal execution of a woman to be carried out in the US in nearly 70 years.
Lisa Montgomery was killed by lethal injection in Indiana after the US Supreme Court voted 6-3 in favour of lifting a last minute stay of execution.
Her execution came just days before President Donald Trump leaves office and made her the eleventh person to be put to death since his administration successfully fought to resume federal executions in July following a 17-year hiatus.
Montgomery, 52, murdered an expectant mother to steal her baby, though she had sought a stay of execution on the grounds she was mentally ill.
A spokesman for the EU's foreign policy chief Josep Borrell said that Brussels "deeply regrets" the execution.
"The EU had called for clemency to be granted to Lisa Montgomery taking into consideration international law and internationally accepted minimum standards that should be respected regarding people with mental disorders," the statement said.
"The EU calls on the US administration to reverse its decision to carry out the remaining federal-level executions during the last days of the current administration."
In a statement, the US Justice Department said: "Lisa Montgomery, 52, was executed at the US Penitentiary Terre Haute in accordance with the capital sentence unanimously recommended by a federal jury and imposed by the US District Court for the Western District of Missouri in 2007."
Montgomery's execution marked the first time the US government has implemented the death sentence for a female prisoner since 1953.
Challenges were fought across multiple federal courts on whether to allow execution of Montgomery, who had initially been scheduled to be executed by lethal injection yesterday in the Justice Department's execution chamber at its prison in Terre Haute, Indiana.
Kelley Henry, Montgomery's lawyer, had called the pending execution, "vicious, unlawful, and unnecessary exercise of authoritarian power".
"No one can credibly dispute Mrs Montgomery's longstanding debilitating mental disease - diagnosed and treated for the first time by the Bureau of Prisons' own doctors," Ms Henry had said in a statement.
Montgomery was convicted in 2007 in Missouri for kidnapping and murdering Bobbie Jo Stinnett, then eight months pregnant.
Montgomery cut the baby from Ms Stinnett's womb. The child survived.
Some of Ms Stinnett's relatives travelled to witness Montgomery's execution, the Justice Department said.
Montgomery's lawyers asked for clemency last week, saying she committed her crime after a childhood in which she was abused and repeatedly raped by her stepfather and his friends. They said she should instead face life in prison.
It is one of three executions the US Department of Justice had scheduled for the final full week of President Donald Trump's administration.
Two other executions scheduled for tomorrow and Friday have been delayed, for now at least, by a federal judge in Washington, to allow them to recover from Covid-19.
Federal executions had been on pause for 17 years, and only three men had been executed by the federal government since 1963, until the practice was resumed last year under Mr Trump, whose outspoken support for capital punishment long predates his entry into politics.