"An astonishing 694 people" were killed by the Iranian state between January 1 and July 15, said the report published Thursday on the Amnesty International website.

"Iran's staggering execution toll for the first half of this year paints a sinister picture of the machinery of the state carrying out premeditated, judicially-sanctioned killings on a mass scale," Said Boumedouha, deputy director Amnesty's Middle East and North Africa program, said in a statement also published online.

Boumedouha warned that Iran was on track to execute more than 1,000 people in 2015, denouncing the country's "deeply flawed" judicial system and the "arbitrary" nature with which death sentences were handed down. The report also said Iran was withholding information as to how many convicted criminals had been executed.

Officially, the Iranian government has acknowledged 246 executions between January 1 and July 15.

In 2014, Iran said that it had executed 289 people. Amnesty said it could confirm 743 executions.

Drug charges

The spike in executions, Amnesty said, was due to an increase in drug-trafficking convictions.

"For years, Iranian authorities have used the death penalty to spread a climate of fear in a misguided effort to combat drug trafficking, yet there is not a shred of evidence to show that this is an effective method of tackling crime," said Boumedouha.

According to an ongoing investigation carried out by Amnesty, several thousand more people are on death row in Iran, with up to 80 percent on drugs charges.