China executed 2,400 prisoners last year, a San Francisco-based rights group said, shedding rare light on a statistic Beijing tries to keep a state secret.

The Dui Hua Foundation said on Monday that the figure had decreased by 20 percent since 2012, but China is so sensitive about the issue that it has done nothing to publicize the decline.

The foundation, a nonprofit organization that seeks clemency and better treatment for at-risk detainees in China, said it obtained its figures from "a judicial official with access to the number of executions carried out each year."

"China currently executes more people every year than the rest of the world combined, but it has executed far fewer people since the power of final review of death sentences was returned to the (Supreme People's Court) in 2007," Dui Hua said.

The foundation estimates that China will execute an additional 2,400 prisoners before 2015.

China's top court examines all death sentences issued in the country. Between July 2013 and Sept. 2014, the court published over 150 death penalty review decisions, with the majority handed down for murder and drug cases, according to Dui Hua.

Human rights groups regularly condemn China for its high execution rate and a death penalty process cloaked in secrecy. Rights group Amnesty International, for example, says death row inmates in China are subjected to unfair trials and convicted for non-lethal crimes like drug trafficking and economic offenses.

Despite declining executions in the Chinese prison system the government’s "use of capital punishment in anti-terrorism campaigns in Xinjiang and the anti-corruption campaign nationwide" will offset any decrease this year, Dui Hua said.

Amnesty releases annual reports of death penalties in 22 countries. Its 2013 report recorded an increase of 14 percent in executions worldwide from the 2012 figure.

However, Amnesty has not published statistics on China's death penalties since 2009 due to the difficulty of obtaining information.

Amnesty's overall figure in 2013 for death penalties carried out worldwide was 778, which means that if the 2,400 figure is accurate, China executed more than three times the number of people than every other country combined.

Outside of China, almost 80 percent of all 2013 executions were recorded in Iran, Iraq and Saudi Arabia. Iran is reported to have executed 369 people, and Iraq is recorded as having executed 169, according to Amnesty.

At least 23,392 people were sentenced to death worldwide by the end of 2013, the rights group said, with most of the sentences related to drugs offenses.

The United States remains the only country in the Americas that carries out executions, with the state of Texas accounted for 41 percent of them in 2013.

According to Amnesty International, by 2013, 173 of the 193 member states of the United Nations are execution free.

Al Jazeera and wire services