Tennessee death row inmate Christa Pike has been taken to the hospital after she was still alive following two lethal injections during her execution, her lawyer says.

Her legal team filed an emergency motion to halt the execution as it was happening, saying: "Ms Pike has lost consciousness and still has a heartbeat and is audibly snoring."

One of her attorneys told the BBC that Pike, 50, was "being provided life-saving measures" in hospital after two syringes of pentobarbital had been administered.

There has been no comment yet from the state of Tennessee. Pike was sentenced to death in 1996 for the murder of Colleen Slemmer.

The attempted execution went ahead after the US Supreme Court overturned a reprieve granted by a lower court on Wednesday morning.

Justice Brett Kavanaugh, a conservative, granted the application to vacate the stay of the execution. No explanation was provided in the order.

The top court's three liberal justices - Sonia Sotomayor, Elena Kagan and Ketanji Brown Jackson - dissented.

Sotomayor wrote that the decision cleared the way for Tennessee to execute Pike before her pending legal challenges to her conviction "can be properly considered".

"Tennessee's desire to expedite Pike's execution by a few days or even weeks cannot outweigh her 'fundamental interest in [her] own life," she said.

The state's order for the execution at Riverbend Maximum Security Institution in Nashville was due to expire at midnight.

After the attempted execution, Dr Joel Zivot, a professor at the Emory University School of Medicine, said Pike likely never achieved a sufficient blood level of pentobarbital to cause apnoea and cardiovascular failure.

"It's very possible that as a consequence of the delay of the beginning of resuscitation she will have a brain injury," Zivot, an expert on physician participation in lethal injection, said.

In a ruling earlier on Wednesday, judges from the US Court of Appeals for the Sixth Circuit paused the execution saying they needed more time to consider the arguments in Pike's case.

Pike's lawyers said that for years, the state of Tennessee had argued in court that she lied about being sexually abused as a child.

But at a recent hearing about her execution method, the state said it did not dispute that she had suffered sexual abuse and rape.

Pike's legal team asked the appeals court to send her motion back to the district court, arguing that if the federal courts had based their ruling on the state's earlier claim that she was lying, then the federal review was affected.

It was the second time in as many days that the US Supreme Court gave the go-ahead for the convicted murderer's death by lethal injection. On Tuesday, the justices rejected a different appeal from Pike.

Tennessee's governor also denied a clemency request this week from the death row inmate.

At the time of her incarceration, the 21-year-old was the youngest person on death row, according to the Tennessee Department of Corrections.

Pike is the only woman currently serving a sentence on Tennessee's death row.

Since 1976, when the Supreme Court resumed the death penalty, only 18 women have been executed in the US, compared to 1,663 men. That's about 1% of all US executions.

Pike was 18 when she and her then-boyfriend, Tadaryl Shipp, beat, tortured and killed 19-year-old Colleen Slemmer, whom they met at a job training camp for troubled teens.

In her clemency petition, Pike said that she intended to beat Slemmer, not kill her, but that she was unable to "put the brakes on".

"I was a mentally ill 18-year-old kid," she said in a statement. "It took me numerous years to even realise the gravity of what I'd done. Even more to accept how many lives I affected. I took the life of someone's child, sister, friend. It sickens me now to think I had the ability to commit such a crime."

The murder in 1995 created a media frenzy in Knoxville, Tennessee, after Colleen was found with an inverted pentagram - a symbol that is associated with satanic groups - carved into her chest. Pike has denied carving the symbol herself.

Before the killing, Pike had accused Colleen of insulting her and trying to steal her then-boyfriend.

Two other residents at the camp later testified that Pike had bragged about the murder before and after, and showed them a piece of Colleen's skull.

Colleen Slemmer's mother has said she wants the execution to take place, telling the Associated Press she has waited decades to see Pike's sentence carried out.

"Every time I think about it, I think about Colleen feeling that pain and trying to get up and run," May Martinez told the agency on Saturday.

She said a group helped raise funds for her and her husband to travel from Florida to Tennessee to witness the execution.

"Not a day goes by, or a minute goes by that I do not think about Colleen. Holidays are the worst," Martinez said.