Washington (AFP) - Oklahoma was poised Friday to become the first US state to use nitrogen gas as an alternative to lethal injection when carrying out executions.

Legislation to implement the measure was on the desk of Governor Mary Fallin after it was approved Thursday in the state Senate by a unanimous vote.

It's seen as giving the south-central state a fallback option if the US Supreme Court rules against its controversial use of lethal injection.

"The person will become unconscious within eight to 10 seconds and die a few minutes later," said Mike Christian, who initiated the bill in the lower House of Representatives, which passed it in March.

"In other words, a humane, quick and painless death," he said.

The US Supreme Court is due later this month to consider the constitutionality of lethal injection, the most common form of execution in the nation.

It upheld the procedure in a 2008 ruling.

But since then, shortages of the most commonly used drugs prompted state officials to come up with new lethal "cocktails" suspected of causing pain and suffering during some recent executions.

The case now before the Supreme Court is being brought by three death-row inmates in Oklahoma, who are challenging an untested triple combination of drugs used in earlier botched executions.