Amnesty International has urged Saudi Arabia to abandon what it termed a "bloody execution spree", after reports that 14 more men are set to be executed.

The kingdom has already beheaded at least 66 people this year, the rights group said on Monday, as it decried the Saudi Arabian supreme court's decision to uphold death sentences against the group convicted of crimes related to their involvement in protests.

Amnesty also said it learned on Sunday that the Specialised Criminal Court (SCC) in Riyadh upheld the death sentences for 15 other men accused of spying for Iran.

In June 2016, the SCC sentenced to death the 14 men charged with crimes relating to protesting in what Amnesty says was a "grossly unfair mass trial".

They were convicted of a wide array of crimes that include bomb making, theft, robbery, participation in riots and shooting at security vehicles, among others.

Court documents showed that the men claimed they had been tortured until they confessed to the allegations.

"By confirming these sentences, Saudi Arabia's authorities have displayed their ruthless commitment to the use of the death penalty as a weapon to crush dissent and neutralise political opponents," Samah Hadid, Director of campaigns for the Middle East at Amnesty International, said in a statement.

"King Salman's signature is now all that stands between them and their execution. He must immediately quash these death sentences which are a result of sham court proceedings that brazenly flout international fair trial standards."

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Amnesty added that Saudi authorities had executed at least 26 people over the past three and a half weeks - an average of more than one execution a day.

Meanwhile, as of Wednesday, the UK-based rights group Reprieve had obtained more than 32,000 signatures on a petition to cancel the executions.

'Utterly shocking'

Earlier this month, Reprieve lambasted Saudi Arabia for upholding a death sentence for 23-year-old Munir Adam, a partially deaf and blind man who was allegedly tortured into a confession.

Police are accused of torturing Adam, who was arrested during protests in 2012, until he lost all hearing in one ear, despite medical records proving his disabilities.

The group called on US President Donald Trump, who visited Saudi Arabia in May, to intervene on Adam's behalf.

"Munir's case is utterly shocking - the White House should be appalled that our Saudi allies tortured a disabled protester until he lost his hearing then sentenced him to death on the basis of a forced 'confession'," Maya Foa, director of Reprieve, said in a statement at the time.

Foa went on to accuse Trump of having "emboldened the kingdom to continue the torture and execution of protesters".

On July 11, Saudi authorities executed four Shia Muslim men convicted on charges of "terrorism" for alleged attacks against police and participating in protest-related violence against security forces.

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On January 2, 2016, the kingdom executed 47 prisoners convicted of various offences, among them prominent Shia religious leader Sheikh Nimr al-Nimr, who had led protests against the government and its Sunni religious leaders.

Nimr's execution prompted demonstrations in Saudi Arabia's Eastern Province and other countries of the Middle East.

In Iran, protests turned violent with demonstrators storming the Saudi embassy, prompting Riyadh to cut diplomatic ties with Tehran.

Saudi Arabia has one of the world's highest rates of execution.

Minority Shia Muslims have long complained of marginalisation and discrimination in the Sunni-ruled kingdom, and demand political and economic reforms.