China has halved its executions since 2007, when its high court began reviewing death row cases, but still puts around 4,000 people to death every year, a US campaign group said Tuesday.

The exact number of people executed in China every year is a state secret, but according to Amnesty International, the country puts more people to death than the rest of the world put together.

The rare data, compiled by San Francisco-based campaign group Dui Hua, is partly based on a claim by a Chinese legal scholar at the quasi-governmental think tank, the Chinese Academy of Social Sciences, that executions have been halved.

It comes in the same week China executed a South African woman by lethal injection for drug smuggling after rejecting last-minute pleas for clemency from her government.

Dui Hua executive director John Kamm said the figure, which is nearly eight times the 527 Amnesty International says were executed outside China in 2010 -- was still far too high.

"China has made dramatic progress in reducing the number of executions, but the number is still far too high and declining far too slowly," he said.

"At the present rate of decline it will take many years for the government to reach its goal of abolishing the death penalty.

"When officials and the public know the full extent of the death penalty in China, abolition will be achieved more quickly."

Beijing has taken measures in recent years to rein in the use of capital punishment, including requiring the country's supreme court to review all such sentences before they are carried out.

Most executions are imposed for violent crimes such as murder and robbery, state media have said, but drug trafficking and some corruption cases are also punishable by death.

Earlier this year, China eliminated capital punishment for some economic crimes, including tax fraud, as it moved to curb use of the death penalty.

The amendment, which took effect on May 1, also exempted from capital punishment anyone over the age of 75 at the time of trial, unless they had committed murder "with exceptional cruelty".

Previously, only convicts younger than 18 or pregnant at the time of trial were exempt.

Executions in China have traditionally been carried out by shooting, but lethal injections are increasingly being used.