The number of executions worldwide dropped by 37 percent in 2016 compared with the year before, mainly because Iran hanged fewer people, Amnesty International said in its 2016 global review of the death penalty published on Tuesday.

China executed more people than all other countries in the world put together, Amnesty said.

But it is difficult to get a clear number as Beijing classifies most information related to the death penalty as "state secrets". It is estimated to be in the thousands each year.

"China wants to be a leader on the world stage, but when it comes to the death penalty it is leading in the worst possible way - executing more people annually than any other country in the world," said Salil Shetty, secretary general of Amnesty International.

"The Chinese government has recognised it is a laggard in terms of openness and judicial transparency, but it persists in actively concealing the true scale of executions. It is high time for China to lift the veil on this deadly secret and finally come clean about its death penalty system."

Excluding China, states around the world executed 1,032 people in 2016, according to Amnesty's records.

The vast majority of those executions - 856 - were carried out in the Middle East and North Africa (MENA).

Historic low in US

Meanwhile, the US reached a historic low in its use of the death penalty in 2016.

For the first time in a decade the US was not one of the five biggest executioners, giving hope to human rights activists. Still 20 people were put to death there, mostly by lethal injection.

Another 2,832 people are still on death row in the US.

Amnesty said the decrease in executions was partly because of litigation on lethal injection protocols and challenges in sourcing chemicals in several states.

The group added that the possible resolution of some lethal injection challenges could see the level of executions start to take off again in 2017 - including in the state of Arkansas, where the governor scheduled eight executions during a 10-day period this April.

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At least 856 executions were carried out across the MENA region in 2016, a drop of 28 percent from 2015 which had seen a sharp increase from previous years.

Iran executed at least 567 people alone, accounting for 66 percent of all the confirmed executions in the region.

Saudi Arabia carried out at least 154 executions in 2016 - just four fewer than the record high of 158 executions in 2015, which was the highest recorded figure since 1995.

In Egypt the number of executions doubled from 22 in 2015 to 44 in 2016 - ranking the country sixth place worldwide.

That increase came as President Abdel Fattah el-Sisi cracked down on political rivals, including the outlawed Muslim Brotherhood. Amnesty said "Egypt has witnessed an unprecedented increase in mass death sentences after unfair trials".

"Many MENA states justify their use of the death penalty by claiming that they are acting to counter grave security threats, despite there being no evidence that the death penalty deters violent crime," said James Lynch, head of the death penalty team at Amnesty International.

"The reality is that many of those executed across the region are from poor and marginalised communities, in hundreds of cases sentenced to death for non-violent crimes."