Saudi Arabia on Thursday beheaded a convicted Pakistani drug trafficker, adding to what a United Nations rapporteur called a “very disturbing” surge in the kingdom's use of the death penalty.

Ihsan Amin was executed in Riyadh after being convicted of heroin smuggling, the interior ministry said in a statement carried by the official Saudi Press Agency.

He became the 90th person put to death this year, compared with 87 for the whole of 2014, according to AFP tallies. About half of those executed have been foreigners.

Also read: Saudi beheads Pakistani, 84 executions in 2015

Rights group Amnesty International called the toll unprecedented and said it was “one of the highest recorded by the organisation during the same period for more than three decades”.

“With the year yet to pass its midpoint, the Gulf kingdom has raced towards this shocking toll at an unprecedented rate,” Amnesty's Middle East and North Africa deputy director Said Boumedouha said.

“This alarming surge in executions surpasses even the country's own previous dreadful records. “Almost half of this year's executions were for drug-related offences, London-based Amnesty added.

“These do not fall into the category of 'most serious crimes', and the use of the death penalty for such offences violates international law,” it said.

The rise in Saudi executions is “out of line” with global trends where the numbers of executions and states which apply the death penalty are decreasing, Christof Heyns, the UN special rapporteur on extrajudicial, summary or arbitrary executions, told AFP on Wednesday.

Under the conservative kingdom's strict version of Islamic sharia law, drug trafficking, rape, murder, armed robbery and apostasy are all punishable by death.

The interior ministry has cited deterrence as the reason for carrying out the punishment.