The White House said Friday it was “deeply troubled” after Alabama executed a man using nitrogen gas in a first-of-its-kind capital punishment.

“The reports of Kenneth Smith and his death last night obviously is very troubling. It is very troubling to us as an administration, it is very troubling to us here at the White House,” press secretary Karine Jean-Pierre told reporters.

“The president has long said and has had deep deep concerns with how the death penalty is implemented and whether it is consistent … with our values,” she added.

Smith was put to death Thursday in Alabama night after breathing nitrogen gas through a face mask, causing oxygen deprivation. It was the first time the method had been used to execute an American prisoner.

Smith had been convicted in a murder-for-hire killing that took place in 1988.

Critics argued the use of nitrogen gas violated the Constitution’s prevention of cruel and unusual punishment.

The Supreme Court rejected a last-minute bid Thursday to block Smith’s killing, but Justice Sonia Sotomayor wrote in a dissenting opinion that Alabama was using Smith as a “guinea pig” to test a new method of execution.

Attorney General Merrick Garland in 2021 imposed a moratorium on the federal death penalty, which the White House said President Biden supports.

“The president has always had deep concerns with how the death penalty is implemented,” Jean-Pierre said.

Despite the moratorium, federal prosecutors said earlier this month they will seek the death penalty against the gunman who killed 10 Black people at a Buffalo, N.Y., grocery store in 2022.