The number of United States (US) executions fell to a quarter-century low in 2016 as new death sentences plummeted, indicating capital punishment is on the decline, a study released on Wednesday showed.

The number of US executions in 2016 was 20, the lowest since 1991, according to the study from the Death Penalty Information Centre, which monitors capital punishment.

While 31 states have the death penalty, only five held executions in 2016. Georgia carried out the most at nine while Texas was next at seven, it said.

The number of new death sentences in 2016 is expected to hit 30, a low not seen since the US Supreme Court declared existing death penalty statutes unconstitutional in 1972, it said. That figure is down by more than 90 per cent from a recent high of 315 in 1996.

Legal battles and a sales ban on execution drugs will likely hold down the number of executions next year while the high costs of death penalty cases is set to keep capital punishment prosecutions down as district attorneys instead seek sentences of life in prison without parole, legal experts said.

"America is in the midst of a major climate change concerning capital punishment," said Robert Dunham, the center's executive director and the report author.

States have been scrambling to find drugs since European drug makers imposed a sales ban about five years ago due to ethical concerns. The problem was exacerbated when pharmaceutical giant Pfizer imposed a sales ban this year, cutting off the last major US source of the drugs.

Ohio, which has executed 53 inmates since the US Supreme Court reinstated the death penalty in 1976, had a US judge this week delay plans to end its nearly 3-year execution hiatus in 2017 to examine its drug procurement secrecy.

Capital punishment advocates have said expenses or drug shortages should not be a factor, arguing the death penalty is an instrument of justice and must be used for those who deserve it.

Jordan Steiker, a University of Texas Law School professor and director of its Capital Punishment Center, said states looking to resume executions are going to face stiff legal challenges.

"We are on a path toward constitutional abolition. The length of that path will be dictated by uncertainties concerning the Supreme Court's composition and how much the withering of the death penalty continues," he said.