At least 690 prisoners were executed in 20 countries in 2018, marking a 31 percent fall from at least 993 the previous year, according to Amnesty International that reported the fewest deaths by capital punishment in at least a decade.

A total of 2,531 death sentences were also imposed in 54 countries last year, decreasing from 2,591 recorded worldwide in 2017 according to a report published on Wednesday by the UK-based rights group that hailed the significant decline in global executions.

As in previous years, China, where figures believed to be in the thousands are classified as a state secret, remained the most prolific executor, Amnesty said, followed by Iran, Saudi Arabia, Vietnam and Iraq - accounting for 78 percent of total reported executions.

"It's very encouraging that known executions were the lowest at the end of 2018, but it is challenging to define clear reasons for the decreases and whether these reductions will be sustained in the coming years," Oluwatosin Popoola, Amnesty International adviser on the death penalty, told Al Jazeera.