An Alabama death row inmate is expected to become the first person in the US to be executed with nitrogen gas, after losing last-minute appeals.

The US Supreme Court and a lower appeals court declined to block what Kenneth Eugene Smith’s lawyers called a “cruel and unusual” punishment.

Opponents say using nitrogen could cause unnecessary suffering, and a leak could harm people present in the room.

Smith, 58, was convicted in 1989 of murdering Elizabeth Sennett.

Alabama has 30 hours to carry out the execution, which involves pumping nitrogen gas through a mask, from Thursday at 0600 GMT (0100 ET).