Media playback is unsupported on your device Media caption Convicts are strapped to a chair before they are executed by marksmen, as Ben Bland reports

Utah will resume the use of firing squads to carry out the death penalty when lethal injections drugs are not available.

Utah Governor Gary Herbert signed the measure into law on Monday.

The move makes Utah the only US state to use firing squads as a method of execution.

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

Drug inventories dwindled after European manufacturers opposed to capital punishment refused to sell the lethal concoctions.

Civil rights groups have said use of firing squads makes Utah "look backward and backwoods".

Governor Herbert finds the firing squad "a little bit gruesome,'' but said the state needs a back-up execution method.

"We prefer to use our primary method of lethal injection when such a sentence is issued," the governor's spokesman Marty Carpenter told the Associated Press.

"However, when a jury makes the decision and a judge signs a death warrant, enforcing that lawful decision is the obligation of the executive branch."

Last execution

Media playback is unsupported on your device Media caption The five ways the US executes - in 45 seconds

It will probably be years before Utah's next execution. The head of Utah's prison system has said the state does not have any reserves of lethal injection drugs.

The new Utah law reinstates the use of firing squads more than a decade after the state abandoned the practice.

Because of the intense media attention, Utah lawmakers stopped offering inmates the choice of a death by firing squad several years ago.

But a handful of inmates sentenced to death before 2004 still have the option of going before a firing squad.

Ronnie Lee Gardner, a convicted murderer who shot and killed a lawyer in attempt to escape from prison, was the last inmate executed by a firing squad in 2010.