There were more executions worldwide in 2015 than in any year since 1990 and almost 90 percent occurred in three countries - Iran, Saudi Arabia and Pakistan, human rights watchdog Amnesty International said on Wednesday.

At least 1,634 people were executed last year, the organisation said, adding that the actual number was probably significantly higher given that there are no definitive numbers for China.

"The number of known executions rose by more than 50 percent compared with 2014 - this development is unsettling and alarming," said Oliver Hendrich, an expert on capital punishment at Amnesty International in Germany.

At least 977 people were executed in Iran last year, mostly for drug crimes, Amnesty said, while more than 320 death sentences were carried out in Pakistan and at least 158 people were executed in Saudi Arabia.

In the United States, 28 people were executed last year - the lowest number since 1991, Amnesty said.

Countries that impose the death penalty are in the minority for the first time now, Amnesty said. It added that 102 countries had got rid of the death penalty for all crimes by the end of 2015, compared with 60 countries in 1996.