Hong Kong (CNN) The world's most prolific executioner killed "thousands" of people last year, human rights group Amnesty International estimates.

But that figure does not include China, still believed to be by far the world's top executioner.

Lack of transparency

While Amnesty does not publish figures on China due to concerns over how the government has used its figures in the past -- which the rights group said were far lower than reality -- the report said "available information indicates that thousands of people are executed and sentenced to death in China each year."

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Death Penalty Worldwide, a research and advocacy group under Cornell Law School, estimates that there were at least 2,400 executions in China in 2014, or one execution per 562,500 persons.

"It's still very opaque," William Nee, Amnesty's China researcher, told CNN.

This lack of transparency is confusing, said Nee, given the most common justification given for the death penalty.

"One of the paradoxes about the death penalty in China, is if it's supposed to be a deterrent, you would think all death sentences should be made public."

On the contrary, state media and other coverage of executions has actually reduced, according to Nee.

Slow progress

While the number of executions has remained high, gradual reform has taken place, with the number of capital crimes being reduced by 13 in 2011, and by a further nine last year.

However, several of the crimes de-listed, including smuggling nuclear material, or "fabricating rumors to mislead others during wartime," were very rarely prosecuted.

"In practice it's not really that much of a difference," Nee said. "But it does show a determination on the part of government to slowly decrease the use of the death penalty."

Public opinion shift

18-year-old Huugjilt was executed for a crime Zhao Zhihong (center) later confessed to.

A number of prominent botched convictions and executions have also sparked greater opposition to and criticism of the death penalty in China.

In February, 27 officials were penalized over the wrongful execution of an Inner Mongolian teenager.

In 1996, 18-year-old Huugjilt was convicted of the rape and murder of a woman and executed in a matter of months. Nine years later, convicted serial rapist and murderer Zhao Zhihong confessed to the crime, sparking a review of the case and intense media coverage.

Inner Mongolia's high court overturned Huugjilt's conviction in December 2014, and gave his parents 2 million yuan ($320,000) in compensation for the wrongful execution of their son.

"We learned a heartbreaking lesson in this case; we are sorry," said Zhao Jianping, the court's deputy president, in overturning the verdict.

While it is "impossible to have a widespread public movement" against the death penalty in China due to government controls on free expression, Nee said there has been a general shift in attitudes against capital punishment.

However, he warned that without thorough reporting, any progress is difficult to measure.

He pointed to pollution statistics as some that was regarded as a state secret not long ago and is now publicly reported.

"The Chinese government should fully come clean about the truth of the death penalty in China."