A surge in executions by Saudi Arabia is "very disturbing" and bucks the global trend, a UN special rapporteur told AFP on Wednesday.

He spoke as the number of beheadings in the kingdom this year hit 89, compared with 87 during all of 2014.

"It is certainly very disturbing that there is such a fast pace of executions at the moment," Christof Heyns, whose mandate includes extrajudicial, summary or arbitrary executions, said in a telephone interview from South Africa.

"If it continues at this pace we will have double the number of executions, or more than double the number of executions, that we had last year."

Heyns, who submits annual reports to the UN Human Rights Council and General Assembly, said Riyadh's use of the death penalty "is just so way out of line" with global trends where the number of executions and states which apply the death penalty is decreasing.

"So this is going in the opposite direction. It's going against the stream," said Heyns, a professor of human rights law at the University of Pretoria.

Statistics indicate that Saudi Arabia last year had the world's third-highest number of executions after China and Iran, he said, adding there are a number of concerns about the kingdom's use of the death penalty.

Under international law, if capital punishment is imposed at all it should only be for murder, he said.

But in Saudi Arabia "more than half" the executions are for non-lethal crimes.

Under the Gulf nation's strict version of Islamic sharia law, drug trafficking, rape, murder, apostasy and armed robbery are all punishable by death, as are other offences including espionage.

In a country of about 29 million, a "very high number of people for the population" are sentenced to death and executed, the special rapporteur said.

"It seems that many of these trials are in secrecy and that lawyers are not available and they do not comply with the standards of fair trial."

The kingdom has, however, pledged to increase the transparency of legal proceedings related to death sentences, Heyns said.

Saudi Arabia's interior ministry has cited deterrence as a reason for implementing capital punishment. Executions are carried out in public, mostly by beheading with a sword.

In the latest case, Fahd bin Hussein Daghriri, a Saudi, was executed on Wednesday in the southern region of Jazan.

He had been found guilty and sentenced to death for his role in the murder of a fellow Saudi citizen, the interior ministry said in a statement carried by the official Saudi Press Agency.

Heyns said it is hard to explain the kingdom's surge in executions, around half of which involve foreigners.

An increase began towards the end of the reign of King Abdullah, who died on January 23.

It accelerated this year under his successor King Salman, in what the watchdog Amnesty International has called an unprecedented "macabre spike".

Salman has adopted a more assertive foreign policy, and in April promoted his powerful Interior Minister Mohammed bin Nayef to be crown prince and heir to the throne.

Heyns said specific reasons for the surge are hard to pinpoint.

Those beheaded this year include Siti Zainab, an Indonesian domestic worker convicted of murder despite concerns about her mental health, according to the Indonesian newspaper Kompas.

Jakarta summoned Riyadh's ambassador over her case, a rare diplomatic incident linked to Saudi Arabia's executions.

Heyns said governments should "make an issue" of their citizens facing death sentences.

"International scrutiny makes it unsustainable for states to continue" with capital punishment, he said, calling it a practice increasingly viewed as "something which really doesn't belong in the 21st century".