The number of known executions worldwide rose to at least 778 last year following a surge in Iraq and Iran, Amnesty International said Thursday, but China remains the world's biggest state executioner by far.

Beijing is thought to have killed thousands of its own citizens, more than the rest of the world put together, the London-based human rights organisation said.

But the charity's annual report on death sentences and executions worldwide said the Chinese authorities "continue to treat the figures on death sentences and executions as a state secret".

"We need really to spotlight China's secrecy around the death penalty," Audrey Gaughran, Amnesty's director of global issues told AFP.

"The authorities in China said that since 2007 they have reduced the use of the death penalty. So our challenge to them is if you have, publish the data and show us," she said.

Although Beijing said in November it would reduce the number of crimes eligible for the death penalty from the current 55, it still led the top five countries using the death penalty in 2013, followed by Iran, Iraq, Saudi Arabia and the United States.

The rise in the known judicial uses of the death penalty -- from at least 682 in 2012 -- was chiefly due to Iraq and Iran, the report said.

Iran put at least 369 people to death in 2013, up from at least 314 in 2012, and Amnesty said there was credible evidence from sources in the country that at least 335 further executions were carried out in secret.

Iraq executed at least 169 people in 2013, a sharp rise on the 40 given the death penalty in 2011 and 101 put to death in 2010, with death sentences there often passed after "grossly unfair trials", the report said.

"The virtual killing sprees we saw in countries like Iran and Iraq were shameful," said Amnesty secretary general Salil Shetty.

"But those states who cling to the death penalty are on the wrong side of history and are, in fact, growing more and more isolated.

"Only a small number of countries carried out the vast majority of these senseless state-sponsored killings. They can't undo the overall progress already made towards abolition."

- Execution for 'enmity against God' -

People were executed in 22 countries in 2013, one more than the previous year, although Indonesia, Kuwait, Nigeria and Vietnam all resumed use of the death penalty.

But Shetty said that despite this, "the long-term trend is clear -- the death penalty is becoming a thing of the past".

Outside China, almost 80 percent of executions worldwide were carried out by Iran, Iraq and Saudi Arabia.

Only five other countries have executed in each of the past five years: Bangladesh, North Korea, Sudan, the United States and Yemen.

In the US last year, Maryland became the 18th abolitionist state, with Texas now accounting for 41 percent of all executions in the Americas.

In a separate list, of death sentences passed last year, Egypt was eighth, with at least 109, but that figure may swell in next year's report after an Egyptian court Monday sentenced 529 supporters of deposed Islamist president Mohamed Morsi to death -- the largest mass sentencing in modern Egyptian history.

Worldwide, people were executed for murder, drug-related offences, adultery, blasphemy, economic crimes, rape, "aggravated" robbery, treason, collaboration with foreign entities, acts against national security, and, in Iran, enmity against God.

The report said people were executed in Saudi Arabia for crimes committed while they were under 18, and possibly in Iran and Yemen too.

Methods of execution included hanging, beheading, electrocution, shooting and lethal injection. Five Yemeni men were beheaded in Saudi Arabia before their corpses were hung from a pole between two cranes.

"We urge all governments who still kill in the name of justice to impose a moratorium on the death penalty immediately, with a view to abolishing it," Shetty said.