Executions in the US are set to hit a 24 year low - the combination of increased opposition to the death penalty and a struggle to obtain the lethal chemicals used to carry them out.

The last two scheduled executions of the year are set to be carried out later this week, with Texas due to put convicted murderer Raphael Holiday to death on Wednesday and Georgia fixed to execute convicted murderer Marcus Johnson on Thursday.

Reuters said that if those two executions go ahead, there will have been 27 executions in the United States in 2015. That would be the lowest number since 1991.

Dylann Roof did not appear to react as he appeared in court via video link (AP)

The death penalty, which remains legal in 31 states, has been hit by the left and right in 2015, the news agency said. Court battles and a scramble to secure execution drugs after a sales ban a few years ago imposed by makers, mostly in Europe, have left about eight states, most notably Texas, Florida and Missouri, as those that conduct executions. In 1999, 20 states put people to death.

Last year nationwide, there were 73 new death sentences, and that number is set to drop by at least a third this year, according to the Death Penalty Information Centre.

Oklahoma, one of the most active death penalty states, has put a halt on executions after mistakes in protocols that led to a flawed execution in 2014 and the delivery of the wrong drug to the death chamber this year. In September, officiacls in South Carolina revealed they intended to seek the death penalty for Dylann Roof, the man charged with six nine counts of murder over the shooting of nine church members.

In some states, such as Nebraska, conservatives have spoken out over the high cost of executions, with some studies suggesting that when appeals and investigations are taken into account, the cost of putting someone to death can be at least double those of housing an inmate for life and are usually far higher. Data cited by the Marshall Project, a nonprofit newsgroup that focuses on criminal justice, has suggested the costs can sometimes be twice as high.

Texas and Virginia have instituted changes in the way death penalty cases are taken through courts that have led to decreased prosecutions.

Death penalty advocate Robert Blecker, a professor at New York Law School, said money should not be a factor.