Tennessee execution of Christa Pike halted after lethal injection fails to cause death

A woman on death row in the US state of Tennessee is still alive after two doses of lethal injection, according to her lawyers.

Christa Pike was scheduled to be executed at a maximum security prison in Nashville at 10am on September 30, local time.

Her execution was delayed after a last-minute stay, with the Supreme Court ultimately clearing the way for it to go ahead a few hours later.

Her legal team has now said she is still alive more than an hour after the execution drug was administered.

Witnesses have been released from the Riverbend Maximum Security Institution, where the execution was held.

Emergency service teams have been seen outside the prison and reportedly left the area with lights and sirens blaring.

Speaking to media, multiple witnesses from the execution chamber said "nothing we saw was normal".

They noted the process was "abnormally long", with the curtain being drawn at multiple points throughout the lethal injection being administered.

Associated Press journalist Kim Chandler said she had witnessed many executions, but never one in Tennessee.

"I would say that Pike appeared noticeably awake as the execution began," she told media outside the prison.

"The blinds were closed for the first time to the execution chamber at 7:46pm, they opened again at 7:49pm.

"Her mouth fell open a few times and she began to loudly snore. She gasped a few times.

"The curtain to the execution chamber closed for the final time at 8:06pm and she could be heard loudly snoring, clearly very alive, in the chamber until media witnesses were told to leave the area at about 8:53pm."

Prison officials have yet to speak.