The state legislature has passed a bill, which now awaits the governor's signature to become law.

Several US states are considering alternative execution methods as they struggle to obtain lethal injection drugs amid a nationwide shortage.

Inventories dwindled after European manufacturers opposed to capital punishment refused to sell the drugs.

Later this month. the US Supreme Court will review the use of midazolam, a drug used in Oklahoma executions.

Last year a condemned inmate in Oklahoma struggled for almost an hour during a lethal injection execution.

It is the first drug applied to a prisoner during an execution, and is followed by two others which stop the heart and cause death. Alternatives to midazolam are short supply.

Under the new law, nitrogen gas chambers will become Oklahoma's primary backup method of execution if lethal injection drugs are unavailable or ruled unconstitutional by the Supreme Court.

Electrocution and firing squad are the backup methods currently available to the state. The bill was passed by the legislature without a single dissenting vote. A spokeswoman for Governor Mary Fallin, a strong supporter of the death penalty, tells BBC News Online that her decision will be announced next week. Supporters of the bill say nitrogen gas poisoning, or nitrogen-induced hypoxia, is a more humane and painless way to die and requires no medical expertise to perform. There are no reports that it has ever been used before on humans. The governor of Utah signed a bill on 23 March that allows death by firing squad as a backup option should lethal injection drugs become unavailable. The decision has been criticized by the American Civil Liberties Union as "brutal" and "barbaric."Oklahoma has temporarily halted its remaining executions as the state awaits review by the Supreme Court.