The total number of known executions worldwide rose by more than half last year to 1,634, the highest figure recorded since 1989, Amnesty International said Wednesday as Pakistan sent three more men to the gallows.
The surge was largely fuelled by Iran, Pakistan and Saudi Arabia, the London-based human rights organisation said in its annual report on death sentences and executions worldwide.
The 1,634 figure does not include China, which is thought to have killed thousands of its own citizens.
Death penalty data is "treated as a state secret" by Beijing, Amnesty said, as it is by Vietnam and Belarus.
Recorded executions were up by 54 percent on 2014's figure of 1,061.
Just three countries -- Iran, Pakistan and Saudi Arabia -- were responsible for 89 percent of the total of 1,634.
"The rise in executions last year is profoundly disturbing," said Amnesty secretary general Salil Shetty.
"Not for the last 25 years have so many people been put to death by states around the world.
"Iran, Pakistan and Saudi Arabia have all put people to death at unprecedented levels, often after grossly unfair trials. This slaughter must end."
Pakistan lifted a six-year moratorium on the death penalty following a school massacre by Taliban insurgents in December 2014.
Initially it brought back hanging just for terrorist killings but later extended it to all capital crimes.
It hanged three convicted murderers including a pair of brothers on Wednesday, a senior prisons official told AFP.
"Over the past year, Pakistan has vaulted to the number three spot for recorded state executions in the world -- a shameful position no one should aspire to," Champa Patel, director of Amnesty's South Asia office, told AFP, adding the majority were not convicted of terror offences.
Pakistan executed 326 people in 2015 while Saudi Arabia put 158 people to death.
Iran's execution of at least 977 people is at odds with its opening up to the West after striking a deal with world powers last year on its nuclear ambitions, Amnesty said.
"Western countries are starting to build commercial ties and trade missions," said James Lynch, Amnesty's Middle East and North Africa deputy director.
"However, human rights has been absolutely left in the margins," he told AFP. "That risks undermining all these efforts."
For the first time ever, the majority of the world's countries have abolished the death penalty for all crimes.
Fiji, Madagascar, Republic of Congo and Suriname fully abolished the death penalty in 2015, taking the total number of countries to do so to 102.
In China, Amnesty said there were signs that the number of executions had decreased in recent years but it could not verify this.
In August nine crimes were removed from the list of offences punishable by death, bringing the total down to 43.
"Executing several thousand people a year is really very serious and China knows it would be the black sheep of the international community if it was to release the numbers," Nicholas Bequelin, Amnesty's East Asia regional director, told AFP.
People were executed in 25 countries in 2015 by beheading, hanging, lethal injection or shooting.
Amnesty said its reports indicated that four people in Iran and at least five in Pakistan were executed for crimes committed when they were aged under 18.
Worldwide, people were sentenced to death or executed for murder, drug-related offences, corruption, armed robbery, adultery, aggravated rape, rape, apostasy, kidnapping and insulting the prophet of Islam.
A total of 28 people were executed in the United States.
Forms of treason, including "acts against national security", "collaboration" with a foreign entity, "espionage", "questioning the leader's policies", participation in "insurrectional movement" were among those offences worldwide punished with death sentences.
Amnesty recorded a drop in the number of death sentences imposed in 2015 compared to 2014, but said this was partly due to difficulties in corroborating data.
At least 1,998 people were sentenced to death in 61 countries.
At least 20,292 people worldwide were under sentence of death at the end of 2015.