Execution by lethal injection may cause excruciating pain, contradicting its reputation as a humane and thus publicly acceptable way to impose the death penalty, Human Rights Watch said.

Executioners fail to take the steps needed to ensure a painless death and use a drug that veterinarians have deemed too cruel for putting down dogs and cats, the group said in a report released on Monday.

However, a leading death penalty proponent dismissed the report as "blind speculation," saying there was no evidence of someone being conscious and in agony during lethal injection.

Human Rights Watch, which opposes the death penalty in all cases, issued the report amid increased scrutiny of lethal injections across America.

A North Carolina man was executed by lethal injection on Friday by officials who, following a judge's order, used a brain wave monitor to ensure he did not suffer undue pain.

In other states such as Florida and California, executions have been delayed while courts consider whether the lethal injections cause agony. An execution in California was halted in February after the prison failed to find anesthesiologists willing to certify the inmate's death was painless.

"There is mounting evidence that prisoners may have experienced excruciating pain during their executions. This should not be surprising given that corrections agencies have not taken the steps necessary to ensure a painless execution," Human Rights Watch said.

Human Rights Watch does not endorse any alternative method but said as long as the death penalty is legal in the United States it wanted to enforce international human rights laws requiring the least possible suffering.