The US state of Ohio is postponing the execution of seven death row inmates following several botched lethal injection killings across the country last year.

Ohio Department of Rehabilitation and Correction (DRC) said it had put a hold on the executions while it secured new lethal injection drugs. 

Earlier this month, the department revised its drug protocol following the January 2014 execution of Dennis McGuire, who appeared to be gasping for air during the process that lasted 26 minutes, instead of the expected ten minutes.

The drug cocktail used to kill McGuire included the sedative midazolam, which is not approved by the US Food and Drug Administration and has been used in several bungled executions in the United States.

In a statement, the DRC "announced revised execution dates for seven inmates... to continue necessary preparations for the adoption and implementation of the new execution protocol."

"The new dates are also designed to provide DRC adequate time to secure a supply of the new execution drugs."

Six of the seven executions were due to take place in 2015, but as a result of the delay, Ohio will not carry out any executions this year.

Four scheduled for later in 2016 remain in place.

Midazolam has been used in two other botched executions, in Oklahoma in April and in Arizona in July, where inmates showed signs of suffocation, choking and suffering during lengthy killings.

On Wednesday, the US Supreme Court stayed the executions of three death-row prisoners in Oklahoma pending its review of the drugs used by the state for lethal injections.

In 2008, the court ruled that lethal injection was constitutional, and not a violation of the Eighth Amendment's ban on cruel and unusual punishment.

But since then, a majority of states have changed their drug protocols due to a shortage of the products used in the past.